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National Collaborating Centre for Chronic Conditions (UK). Tuberculosis: Clinical Diagnosis and Management of Tuberculosis, and Measures for Its Prevention and Control. London: Royal College of Physicians (UK); 2006. (NICE Clinical Guidelines, No. 33.)

  • This publication is provided for historical reference only and the information may be out of date.

This publication is provided for historical reference only and the information may be out of date.

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Tuberculosis: Clinical Diagnosis and Management of Tuberculosis, and Measures for Its Prevention and Control.

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5Diagnosis

5.1. Diagnosing latent tuberculosis

5.1.1. Clinical introduction

In asymptomatic persons exposure to, and potential infection with, tuberculosis is demonstrated by a positive skin test, or more recently from a positive blood-based immunological (interferon-gamma) test. Those with a strongly positive skin test are then regarded as having been infected with tuberculosis. Of these people presumed infected, there is a 10–15% chance of developing clinical disease at some point in their lives. If a co-morbidity develops which reduces the immune system (see section 10.2), that risk is increased. The majority of exposed persons will kill off the inhaled bacteria, and be left only with a positive skin test as a marker of exposure. About half of those who develop the clinical disease will do so within five years of the initial infection. In cases where a long period elapses between infection and development of disease, dormant bacilli are thought to remain in either the lung or other sites, which can ‘reactivate’ in favourable circumstances for the organism.

Until recently, only TST (Mantoux and Heaf tests) was available to give evidence of exposure. The tuberculin tests had the advantage of being cheap and relatively easy to perform, but suffered from a number of problems. The test results have to be interpreted within a certain timescale, and patients who do not return, or delay returning, will have either no result or a possibly inaccurate one. False positive results can occur because of the sensitising effect on the immune system of either prior BCG vaccination or opportunist environmental mycobacteria. False negative results can occur due to anything reducing immunity, particularly co-infection with HIV, but also treatments such as cytotoxics, or immunosuppression. Extensive tuberculosis (pulmonary or miliary) can itself also temporarily depress the immunity, and can lead to a paradoxically negative TST. More recently, selective immunological (interferon-gamma) tests have been developed using the tuberculosis antigens ‘early secretion antigen target 6′ (ESAT-6 ) and ‘culture filtrate protein 10′ (CFP-10), which are not present in BCG, and are found in only a few species of environmental mycobacteria. These can be done on either cells or cell products derived from whole blood tests. These tests aim to be more specific by removing false positive results, and to be better correlated with latent infection or dormant organisms.

5.1.2. Methodological introduction

There are currently two interferon-gamma immunological tests commercially available for use in the UK: QuantiFERON-TB Gold and T-SPOT.TB. QuantiFERON-TB Gold measures the release of interferon-gamma in whole blood in response to stimulation by ESAT-6 and CFP-10 which are not present in BCG vaccine strains or the vast majority of non-TB mycobacteria. In the T-SPOT.TB test, individual activated ESAT-6 and CFP-10 specific T-cells are enumerated using ELISPOT methodology.

Studies were excluded if the interferon-gamma immunological test under consideration used purified protein derivative as an antigen (as used in the TST) as these tests were phased out to make way for the newer tests based on more specific antigens.

Eleven diagnostic studies were identified which compared interferon-gamma immunological tests with the TST for the diagnosis of latent infection.9–19 One of the studies included an HIV-positive population, but this study was excluded due to methodological limitations.10

It should be noted that three of the studies used an IGT which only included the ESAT-6 antigen.14,16,17 Although such a test is not commercially available, the studies further elucidate the effectiveness of tests based on such antigens compared to the TST.

A recent systematic review20 of interferon-gamma assays in the immunodiagnosis of TB included comparisons with the TST; however this review included studies already considered here and did not attempt a meta-analysis (as meta-analysis methods are not well defined for heterogeneous diagnostic studies). This review has therefore not been considered.

There is no gold-standard test for latent tuberculosis. Diagnosis has in the past been reliant on the TST but this has poor specificity if there has been BCG vaccination or environmental mycobacterial exposure, which can lead to false positive results. In the absence of a gold-standard reference test, it is not possible to measure directly the sensitivity and specificity of a new test for latent tuberculosis. The studies included here have thus focused on whether interferon-gamma immunological tests correlate better with exposure to M. tuberculosis than the TST.

There is a lack of evidence available on the use of these tests in those who are HIV positive, in other immunocompromised individuals and in younger children. Furthermore, there is an issue of the generalisability of non-UK studies.

5.1.3. Evidence statements

Test results and TB exposure

In a UK study17 of healthy adults in a contact tracing clinic, IGT (ESAT-6 ELISPOT assay) results had a strong positive relationship with increasing intensity of contact exposure (OR 9.0 per unit increase in exposure, 95%CI 2.6 to 31.6, p=0.001), whereas TST results had a weaker relationship with exposure (OR 1.9, 95%CI 1.0 to 3.5, p=0.05). (2)

In a study11 of students aged 11–15 years in the UK from the same school as an index case, the odds of a test result being positive for each increase across four stratified exposure groups increased by a factor of 2.78 (95%CI 2.22 to 3.48, p<0.0001) for the interferon-gamma test (ESAT-6/CFP10 ELISPOT assay) and 2.33 (95%CI 1.88 to 2.88, p<0.0001) for the TST. The interferon-gamma test correlated significantly better with increasing exposure across the four groups than the TST (p=0.03). The odds of a positive interferon-gamma test result increased by a factor of 2.51 (95%CI 1.58 to 3.99, p<0.0001) with each week of direct exposure, which was significantly higher (p=0.007) than that for the TST (OR 1.30, 95%CI 1.10 to 1.54, p=0.002). (2)

In contacts of index cases in the Gambia,13 with increasing M. tuberculosis exposure, the percentage of participants who were tuberculin positive and interferon gamma test (ESAT-6/CFP-10 ELISPOT assay) negative increased from 11% of those sleeping in a different house from the index case to 32% of those sleeping in the same room (p<0.001). (3)

In contacts of an index case on an Italian maternity unit,19 the odds for a test result being positive for each increase across four stratified exposure groups (from no discernible contact to household contacts) increased by 1.93 (95%CI 1.11 to 3.35, p=0.020) for the IGT (ESAT-6/CFP-10 ELISPOT assay) but there was no significant correlation for TST. (3)

In Korea where BCG vaccination is mandatory,15 a study found that the odds of a positive test result per unit increase in exposure across four groups, increased by a factor of 5.31 (95%CI 3.62 to 7.79) for the IGT (QuantiFERON-TB Gold) and by a factor of 1.52 (95%CI 1.2 to 1.91) for the TST (p<0.001). (2)

Test results and BCG status

Healthy adults in a contact tracing clinic in the UK,17 had IGT (ESAT-6 ELISPOT assay) results which were not correlated with BCG vaccination status whereas TST results were significantly more likely to be positive in BCG vaccinated contacts (OR 12.1, 95%CI 1.3 to 115.7, p=0.03). (2)

Students aged 11–15 years from the same school as an index case in the UK11 had IGT (ESAT-6/CFP-10 ELISPOT assays) which showed no significant relation with BCG vaccination status, however, BCG vaccinated children were significantly more likely to have higher Heaf grades than unvaccinated children (p=0.002). (2)

In a UK study16 of healthy household contacts and healthy unexposed controls, ESAT-6 peptide-specific interferon-gamma-secreting cells were detected in 85% of the healthy household contacts who were tuberculin positive. None of the healthy control subjects without a history of TB exposure, responded to this IGT even though all unexposed control subjects were BCG vaccinated. (3)

TST negative Australian born medical students (or those born in another low TB prevalence country),14 with no prior BCG, and no known exposure to TB, were BCG vaccinated and then tested again at five months. ESAT-6 stimulated interferon-gamma levels (using ESAT-6 QuantiFERON) were very low or undetectable in all students both before and after BCG vaccination. Of these students, 46% had TST responses of 0 to 4 mm and 54% had responses of ≥5 mm. Thirteen percent had TST results of ≥10mm. Under current Australian guidelines, one student with a 16mm result was defined as having a TST result suggestive of M. tuberculosis infection. (3)

High school contacts in a TB outbreak in Denmark9 who had high exposure to an index case and were not BCG vaccinated, had agreement between TST and IGT (QuantiFERON-TB Gold) results of 93% (95%CI 86 to 100%). This was 95% (95%CI 88 to 102%) in the low exposure group and an overall agreement between the two tests of 94% (95%CI 89 to 99%) in all subjects tested. The kappa value was 0.866, indicating high agreement between the two tests. (3)

In an Italian study19 of contacts of an index case on a maternity unit, IGT (ESAT-6/CFP-10 ELISPOT assay) results were independent of BCG vaccination status. (3)

IGTs were prescribed by hospital physicians for inpatients or outpatients in an Italian study with no influence from the study investigators.12 After excluding indeterminate results, the agreement between IGT (QuantiFERON-TB Gold) and TST results was significantly lower among BCG-vaccinated individuals than in non-vaccinated individuals (41.5% vs. 80.3%, p<0.0001). (3)

In a study of healthcare workers conducted in India18 (where non-tuberculous mycobacteria are highly prevalent), previous BCG vaccination was not associated with TST or IGT (QuantiFERON-TB Gold) positivity. (3)

Indeterminate test results

An Italian study12 found that indeterminate IGT results (QuantiFERON-TB Gold) were significantly over-represented in patients with a negative TST (28.6% vs. 6.6% in tuberculin positive patients, p<0.001) and were more frequent in patients receiving immunosuppressive therapies than in those who were not receiving such treatments (OR 3.35, 95%CI 1.84 to 6.08, p<0.0001). Immunosuppressive therapy was defined as cancer chemotherapy, systemic steroids, or anti-tumour necrosis factor alfa agents at the time of testing. (3)

5.1.4. Health economics

A decision model was used to compare the expected cost-effectiveness of four strategies of testing for latent infection in the context of a contact tracing programme in England and Wales. The strategies compared were:

  • TST
  • IGT
  • TST followed by IGT for patients with a positive TST
  • no test (inform and advise only).

It was assumed that treatment followed current policy: with appropriate therapy for people diagnosed with active TB or testing positive for latent infection, and BCG when appropriate for others. The analysis did not compare different types of skin tests or different types of IGT.

The model is a decision tree, which does not account for the dynamics of disease transmission within the population. Instead, for simplicity, it was assumed that each primary case of active disease is associated with a fixed number of secondary cases. This is probably a reasonable assumption when comparing tests with similar sensitivity, since the absolute difference in false negatives, and hence in opportunities for transmission within the community, will be small. However, estimates of the relative cost effectiveness of contact tracing per se are less robust and should be treated with caution.

Various assumptions were made about the epidemiology and likely concordance with testing and treatment programmes. However, it should be noted that these factors will vary with the context of contact tracing. There is also considerable uncertainty over the relative accuracy of the TST and IGT, as well as over some of the other model parameters. Whenever possible input parameters and assumptions were based on empirical evidence, but some key parameters were estimated by the health economist and GDG.

Cost-effectiveness of testing strategies in contact tracing

The basecase economic analysis suggests that the two-stage strategy (TST/IGT) is within the range usually considered ‘cost-effective’, at around £26,000 per quality-adjusted life-year (QALY) gained. Compared with this, IGT is not cost-effective (over £150,000 per QALY gained). TST is both less effective and more expensive than all of the other options (it is ‘dominated’).

Variation in optimal strategy with context of contact tracing

The results of the economic analysis were highly dependent on the context of the contact tracing scheme – with a higher-risk cohort of contacts, the expected benefits of early diagnosis of active cases, treatment of latent infection, and vaccination will be greater. Below a prevalence of about 10% none of the testing strategies is cost-effective. At intermediate levels of prevalence (between about 10% and 40%), the two-stage TST/IGT strategy is cost effective. Above 40% IGT on its own is the most cost-effective option.

Table 5Cost-effectiveness of diagnostic strategies

Prevalence of infectionStrategyCost (£)Effect (QALYs lost)ICER* (£ per QALY gained)
0No test£310.00409
TST/IGT£580.00394£178,835
IGT£1020.00394(Dominated)
TST£1390.00404(Dominated)
10%No test£1910.02533
TST/IGT£2400.02323£23,351
IGT£2820.02290£126,813
TST£3140.02310(Dominated)
20%No test£3510.04658
TST/IGT£4230.04252£17,575
IGT£4630.04185£60,073
TST£4890.04217(Dominated)
30%No test£5120.06782
TST/IGT£6050.06182£15,553
IGT£6430.06081£38,081
TST£6640.06123(Dominated)
40%No test£6720.08907
TST/IGT£7880.08111£14,522
IGT£8240.07976£27,132
TST£8380.08029(Dominated)
50%No test£8320.11031
TST/IGT£9700.10040£13,898
IGT£1,0050.09872£20,578
TST£1,0130.09936(Dominated)

Uncertainty over optimal testing strategy for contact tracing

The results of the economic analysis were subject to a high degree of uncertainty. The results were very sensitive to assumptions about the relative accuracy of the two types of test, the risk of current and future TB in the cohort, the level of transmission to the wider population, and also to the expected net benefit of avoiding each active case of TB.

5.1.5. From evidence to recommendations

Interferon-gamma tests showed little evidence of being affected by prior BCG vaccination, and showed stronger correlation with exposure categories than did TST. This was shown in low prevalence groups, in household contacts, and in outbreak situations. The specificity of interferon-gamma tests seemed better, and there was less potential for false positive results. It is not possible to determine, for either TST or IGT, the rate of false negative results. Some people with false negative results will go on to develop active TB and thus reduce the cost-effectiveness of vaccination and treatment of latent TB infection.

Prospective studies in people with latent TB (as judged by positive interferon-gamma tests) found at TB contact tracing and new entrant screening, have not yet been performed to find what proportion of such persons went on to develop clinical disease.

Economic modelling was undertaken with various strategies from no action to a two-step strategy with either TST followed by interferon-gamma testing, or serial interferon-gamma tests. Of these options, the model provided most support, on grounds of cost-effectiveness, for a two-step approach with an initial TST, followed by an interferon-gamma test to confirm positivity. The GDG members also supported this because of clinical utility and feasibility.

RECOMMENDATIONS

Evidence is emerging on the performance of interferon-gamma tests. If this new evidence is significant, NICE will consider updating the guideline.

R1.

To diagnose latent TB: [D]

  • Mantoux testing should be performed in line with the ‘Green Book’21
  • those with positive results (or in whom Mantoux testing may be less reliable) should then be considered for interferon-gamma immunological testing, if available
  • if testing is inconclusive, the person should be referred to a TB specialist (see chapter 10 for management of latent TB).

Cross-referring:

For details of people more likely to develop active TB, see section 10.2.

For people with negative tuberculin skin test results, see BCG vaccination under chapter 11.

For people with latent TB, see treatment of latent TB infection under section 10.1.

5.2. Diagnosing active tuberculosis

5.2.1. Clinical introduction

Signs and symptoms of respiratory TB

Primary respiratory tuberculosis is often asymptomatic, but the fact that infection has occurred is shown by the development of a positive tuberculin skin test or interferon-gamma blood test. A history of recent contact with a person with TB is the most important factor in making the diagnosis. Occasionally, tuberculin conversion is accompanied by erythema nodosum or phlyctenular conjunctivitis. Mediastinal nodal enlargement as part of the primary complex can sometimes press on or discharge into a bronchus causing collapse of the distal lung or bronchial narrowing leading to wheeze or obstruction with distal over-inflation.22

In children with primary TB, weight loss, or weight loss and cough, are symptoms associated with culture confirmed TB. However, about half of all children with primary TB will have no symptoms.

Post-primary tuberculosis may be asymptomatic in the early stages, but symptoms, which can be either constitutional or respiratory, soon develop. Malaise, weight loss, fever and night sweats are the common constitutional symptoms. Cough is the commonest respiratory symptom, which is initially dry and non-productive but may later become productive, with haemoptysis in a small minority of cases. Breathlessness is a late feature, usually only occurring when a substantial amount of lung is destroyed or there is a significant pleural effusion. Chest pain is relatively uncommon, but can be pleuritic if peripheral lesions are present, or of dull ill-localised nature.

A study in Sudan, grading sputum smear positivity with clinical features showed multiple chest symptoms were positively correlated with sputum smear positivity. Also, the longer the duration of symptoms, the more this correlated with sputum smear positivity.23 A comparison of the ‘classic’ symptoms of tuberculosis in patients with and without tuberculosis24 is summarised in Table 6.

Table 6. Classic symptoms of tuberculosis.

Table 6

Classic symptoms of tuberculosis.

A multivariate analysis25 showed that the following features were positively associated with culture proven tuberculosis:

  • the presence of TB risk factors or symptoms (OR 7.9)
  • a positive skin test for tuberculosis (OR 13.2)
  • a high temperature (OR 2.8)
  • upper lobe disease on a chest radiograph (OR 14.6)

and that the following were negatively correlated with tuberculosis:

  • shortness of breath (OR 0.2)
  • crackles on physical examination of chest (OR 0.29).

Signs and symptoms of non-respiratory TB

Tuberculosis can affect nearly every non-respiratory site, sometimes with a combination of respiratory and non-respiratory sites, or single or multiple non-respiratory sites.22 As with respiratory tuberculosis, there can be systemic and site-specific symptoms. Weight loss is particularly associated with disseminated (including miliary) and gastrointestinal tuberculosis. Fever and night sweats are common in some non-respiratory sites of disease (disseminated, including miliary, and gastrointestinal TB), but are not common in others (peripheral lymph nodes, skin, bone and joint, genitourinary TB). Tuberculosis has to be considered in the differential diagnosis of an unexplained fever, particularly in those born abroad and/or in ethnic minority groups.

Because of the multiplicity of potential sites of non-respiratory TB, suggestive symptoms are considered site by site.

Signs and symptoms of lymph node TB

Nearly half of all non-respiratory TB in England and Wales occurs in peripheral lymph nodes, mainly cervical.26,27 The nodal enlargement in TB is usually gradual and painless, but can be painful if rapid. The usual absence of erythema and warmth makes the classical ‘cold abscess’. The nodes originally are discrete and firm, but may later mat together and become fluctuant as necrosis develops, which can discharge through the skin with sinus formation and superficial ulceration. Persistent lymphadenopathy of over four weeks duration in people other than white UK-born should be regarded as TB until proven otherwise and investigated appropriately.

Signs and symptoms of bone and joint TB

Bone and joint TB accounts for some 10–15% of non-respiratory disease, with approximately 50% in the spine, and 50% in a wide range of other bones and joints.28,29

With spinal disease pain is the commonest symptom, and may be accompanied by local tenderness or slight kyphosis. Grosser kyphosis occurs when disease has progressed. Paraspinal abscesses can develop and may present as a loin mass, or as a psoas abscess pointing below the groin or causing psoas spasm with hip flexion. Compression on spinal nerve roots can mimic abdominal pathology. Extradural abscess or spinal collapse and subluxation can lead to sensory and motor symptoms involving the legs and sphincters due to spinal cord compression. Back pain and/or neurological signs should have an infective process in the differential diagnosis, particularly in ethnic minority groups.

A wide range of other joints can be involved. TB should be included in the differential diagnosis of unusual bone and joint lesions, particularly of an isolated lesion or a mono-arthritis in an ethnic minority group.

Signs and symptoms of gastrointestinal TB

This form of disease, as with nearly all other non-respiratory sites, is much commoner in ethnic minority groups. The gastrointestinal tract can be involved anywhere along its length, but peri-anal and upper gastrointestinal sites are uncommon (3% of gastrointestinal TB).30 Series in both the developing31 and developed world32 show approximately one third of cases present acutely simulating abdominal emergencies and two thirds with a more gradual onset. Of the cases with an acute onset, approximately one half have right iliac fossa pain simulating acute appendicitis and the other half acute intestinal obstruction. Of those with a more gradual onset of symptoms, fever and malaise, abdominal pain and weight loss are the commonest described symptoms,32 being found in 72%, 60% and 58% of cases respectively in another series.33 Abdominal distension, usually due to ascites, is reported in between 10%32 and 65%34 of cases. There may be right iliac fossa tenderness simulating appendicitis, or a right iliac fossa mass simulating appendix abscess or carcinoma. The ileocaecal area is the commonest site of disease. With bowel involvement there may be acute or sub-acute small bowel obstruction with vomiting and abdominal distension; there may also be palpable mass. The colon distal to the caecum is involved in up to 10% of cases32 and is a cause of gastrointestinal bleeding.35

Signs and symptoms of genitourinary TB

Genitourinary TB is one of the commoner sites of non-respiratory TB in white UK-born people. For example, in 1993 it accounted for 17% of non-respiratory cases in the white UK-born ethnic group, compared with 4% in people of Indian (subcontinent) origin.27 In white cases renal tract lesions predominate but female genital disease predominates in the Indian sub-continent ethnic group.36

Renal tuberculosis is often a ‘silent’ disease with insidious progression which can lead to total unilateral renal destruction. Systemic features such as weight loss, fever and night sweats are not common. As disease progresses, dysuria, haematuria, nocturia and pain either in the loin or anteriorly may occur. Renal disease can lead to ureteric and then bladder involvement by tubercle bacilli seeding distally. Bladder involvement initially leads to cystitis symptoms with frequency and dysuria, but as bladder wall inflammation with associated fibrosis worsens, bladder capacity falls and can be greatly reduced, the so-called ‘thimble bladder’ leading to marked frequency and nocturia due to a tiny bladder capacity. The urine with renal and ureteric disease, but particularly with bladder disease, shows proteinuria and haematuria on dipstick testing, and pus cells on microscopy but is sterile on standard culture. The finding of sterile pyuria should lead to the routine sending of three early morning urines for TB culture. A cold perinephric abscess can occur pointing in either the loin or like a psoas abscess in the groin. Prostatic, epididymal and testicular TB are less common. Testicular TB can present as a mass simulating testicular tumour.

Female genital TB is due to either haematogenous spread or direct spread from intra-abdominal disease. As with urological TB, systemic symptoms are uncommon unless there is associated abdominal tuberculosis. Infertility, either primary or secondary, is the commonest presentation of tubal and endometrial TB.37 Most have no associated symptoms, but menorrhagia is reported in 20–25%, with much lower proportions having amenorrhoea or post menopausal bleeding.37

Signs and symptoms of disseminated (including miliary) TB

Disseminated TB occurs when tubercle bacilli are spread acutely though the blood stream. The symptoms are insidious at the onset with malaise, fever, anorexia and weight loss. In addition, headache from associated TB meningitis can occur with disseminated TB.

Signs and symptoms of central nervous system TB

Although only forming 5% of non-respiratory TB,36 TB of the CNS is of disproportionate importance because of its significant morbidity and mortality. Early symptoms are non-specific with anorexia, malaise, headache, vomiting and altered behaviour. In children these can be poor feeding, irritability, altered behaviour, drowsiness or seizures. The prodromal phase can last from two weeks to two months, then focal neurological signs or decreasing level of consciousness occur. If cranial nerve palsies are present, 3rd and 6th nerve palsies are commoner than 7th and 8th nerve palsies. Internuclear ophthalmoplegia or lateral gaze palsies are less common but more serious because of midbrain or brainstem involvement.37 Other neurological signs can develop depending on the site of endarteritis or infarction, including cerebellar signs, extrapyramidal movements such as choreoathetosis, hemiparesis or monoparesis.

Signs and symptoms of skin TB

Skin involvement can be due to disease of underlying structures, usually lymph node, bone or urogenital tract, with discharge through the skin, with sinus formation, so-called ‘scrofuloderma’. Lupus vulgaris is a slowly destructive local skin form with dull red or violaceous edges. The tuberculides are forms of skin disease thought to be a manifestation of TB elsewhere in the body. Panniculitis, erythema induratum (Bazin’s disease), and papular and papulo-necrotic forms are described and TB is in the differential diagnosis of such lesions, particularly in ethnic minority groups.38

Signs and symptoms of pericardial TB

TB can cause either pericardial effusion or constrictive pericarditis, particularly in ethnic minority groups. Fever, malaise, sweats, cough and weight loss can occur. The signs of pericardial effusion are oedema, pulsus paradoxus, a raised venous pressure, and hypotension with a narrow pulse pressure. With constrictive pericarditis, oedema, abdominal distension and breathlessness are the major signs and symptoms. A lymphocytic exudate on pericardial aspirate should be regarded as TB until proven otherwise.

Signs and symptoms of TB at other sites

TB should be considered in the differential diagnosis of adrenal deficiency, liver abscess, pancreatic mass in young adults with fever, and for isolated ‘cold’ abscesses wherever found, particularly in ethnic minority individuals.

Diagnosing active respiratory tuberculosis

The diagnosis of TB is suspected from a combination of context, symptoms, clinical signs and investigations. The diagnosis is rarely made from a single piece of evidence, and the sensitivity and specificity of individual tests may not reflect the strength of multiple tests or data. Most of the data on the utility of individual tests comes from studies in patients with proven tuberculosis by positive culture. Certain clinical settings are highly suggestive of tuberculosis in ethnic minority groups or recent TB contacts. These are: a pleural effusion which is a lymphocytic exudate, or isolated mediastinal lymphadenopathy, either supported by a positive skin tuberculin test (or interferon-gamma test). These scenarios should be regarded as tuberculosis until proved otherwise and investigated accordingly.

A significant minority of respiratory TB cases however are not bacteriologically confirmed, but are treated on suspicion and regarded as probable cases because of response to specific anti-tuberculosis medication. The guideline aims to advise clinicians on which tests may help if cultures have been, or are subsequently shown to be, negative.

In children, who often have no culture confirmation, scoring systems have been developed to help diagnosis based on context, symptoms, X-ray appearances and other investigations. Some scoring systems are better validated than others.39

Diagnosing active non-respiratory tuberculosis

Most forms of non-respiratory tuberculosis have a lower bacterial load than for pulmonary disease, being so-called pauci-bacillary forms. A relatively very low proportion of cases have positive microscopy for acid-fast bacilli (AFB), and with the lower bacterial loads, even with rapid culture (see section 5.4) it takes longer to obtain positive cultures. With many of the non-respiratory sites, biopsy histology, or, in the case of lymph node disease, needle aspiration cytology, is available well before bacteriology. The finding of caseating granulomas, or granulomas with Langhan’s giant cells on histology or cytology, is very highly suggestive of tuberculosis. A number of other conditions however can cause non-caseating granuloma formation. In the absence of caseation or Langhan’s giant cells, additional tests such as a tuberculin skin test or interferon-gamma test may be needed to assist in diagnosis. Obtaining a sample for culture is important as this confirms the diagnosis and provides the drug susceptibility profile of the organism. One caution is that in children aged under five, particularly if they are of white UK-born origin, granulomatous lymphadenitis is much more likely to be M. avium complex (MAC) than M. tuberculosis. To confirm this, samples are sent for culture, management for M. avium being completely different from M. tuberculosis in this context.40

The yield of histology/cytology depends on tissue sample size, which is much smaller with aspiration cytology than biopsy, and on the level of immune response which generates the histological appearances. In HIV-positive individuals the histological response depends on the level of immunosuppression. With levels of CD4 lymphocytes above 200/μl typical TB histology is the rule, but as the CD4 cell count falls, particularly below 100/μl, less and less granuloma formation occurs, and with profound immunosuppression there may be no cellular histological response at all. In these circumstances however there is an increased likelihood of AFB being seen microscopically. The differential diagnosis in such very immunosuppressed individuals is usually between M. tuberculosis and MAC infection. Polymerase chain reaction (PCR) techniques may help in distinguishing between these infections on AFB microscopy-positive samples (see section 5.3). A similar diagnostic problem can occur when patients with a very low CD4 count are started on highly active antiretroviral therapy (HAART). The rapid fall in HIV viral load and rise in CD4 count allows an immune response to be mounted to either of these organisms, which was not previously possible. Enlargement of cervical and intra-abdominal lymph nodes in particular are described in this context, which is known as the immune reconstitution or IRIS syndrome.

In some cases of non-respiratory tuberculosis, the diagnosis of TB is not entertained in the differential diagnosis, and the doctor, usually a surgeon, does not send any material for culture, instead placing the entire sample in formalin. This then completely precludes any attempt at bacterial culture, although if AFB are seen histologically it still allows PCR-based techniques to be used (see section 5.3). The same histological and cytological criteria apply as in Table 7. Tuberculin skin tests or whole blood interferon-gamma based tests may be needed to assist with histological appearances that are not fully diagnostic.

Table 7. Suggested site-specific investigations in the diagnosis and assessment of non-respiratory TB.

Table 7

Suggested site-specific investigations in the diagnosis and assessment of non-respiratory TB.

5.2.2. Methodological introduction

Diagnosing active respiratory TB: testing while awaiting culture results

Studies were identified which calculated the sensitivity, specificity or predictive value of plain X-ray, sputum smear microscopy and gastric washings when compared with culture as the gold standard for the diagnosis of respiratory TB. Studies on sputum smear microscopy were excluded from review if they were conducted in non-Organisation for Economic Co-operation and Development countries as it was thought that in terms of background levels of mycobacteria and laboratory standards they might not be representative of the UK.

Eight studies examined the diagnostic accuracy of sputum smear microscopy in comparison with culture. Two US studies were excluded for methodological reasons.41,42

Of the six remaining sputum microscopy studies, five were conducted in the US43–47 and one in Turkey.48 Three of these studies reported results for HIV-positive patients or those with AIDS.43,44,47

Four studies were identified which considered the diagnostic accuracy of chest X-ray in predicting culture results. One Danish study included all patients who had a respiratory sample examined for M. tuberculosis during a specified time period,49 a South African study was of paediatric patients suspected of having TB50 whilst two US studies51,52 considered diagnostic accuracy of chest X-ray in those with AIDS/HIV.

Three studies considered the diagnostic accuracy of gastric washings in children.53–55 Two of the studies were performed more than ten years ago in developing countries in populations with a high proportion of malnourished children, thus their applicability to the UK today is highly questionable. A more recent study performed in Cape Town, South Africa55 compared gastric lavage and induced sputum samples from children in terms of their diagnostic yield, reporting how many cases were culture positive, smear positive or both.

Methodological considerations include the following:

  • In terms of sputum smear microscopy, serial testing of sputum samples will increase the sensitivity and specificity of the test.
  • Sensitivity and specificity values are calculated in different ways, either on a patient basis or a specimen basis.
  • Methods used for processing the sputum specimen (including the minimum volume of sputum required and whether the specimen is expectorated or induced) or the method of isolating cultures may differ in various settings.

Generally studies were unblinded (mostly because they were retrospective analyses). Blinding, however, is probably not crucial to avoid bias in the assessment of smear microscopy as the same samples are used for smear and culture and are subject to standardised laboratory procedures and definitions. It was notable that none of the studies identified were performed in the UK.

Diagnosing active respiratory TB if culture results are negative

Two studies56,57 addressed the issue of what other test results might support a positive diagnosis in those with a negative culture for TB but with suspected respiratory TB. In a South African study a group of black male goldmine employees with small lesions in the lung apices on chest X-ray, and a positive skin test but negative sputum culture, were followed up.56 A diagnosis of TB was made if the smear became positive, if the culture yielded M. tuberculosis or if a histological diagnosis was made. A Hong Kong study had a subgroup of patients who had TB diagnosed on the basis of chest X-ray but had negative culture results.57 This group were followed up for future confirmation of TB by culture of M. tuberculosis from sputum, or by radiographic or clinical deterioration.

Methodological issues for consideration are that the gold standard against which diagnostic tests for TB are usually compared is microbiological identification of TB by culture. This is not a perfect gold standard and culture might be negative in TB cases due to ‘pauci-bacillary disease’ (only a small number of M. tuberculosis organisms are present), sampling error or technical problems. In these cases where culture is negative, the standard against which a diagnostic test might be compared could be response to treatment, clinical features or a positive culture in the future. A TB diagnosis in this population would probably be achieved on a case-by-case basis and this has thus not been the subject of many studies.

Diagnosing active non-respiratory TB: testing while awaiting culture results

Studies were searched for which considered the sensitivity and/or specificity of histology from biopsy when compared with culture as the gold standard for the diagnosis of non-respiratory TB. Biopsies could be obtained during surgical procedures or by fine needle aspiration.

Four studies were identified where sensitivity of histology was calculated or it was possible to calculate sensitivity from the results reported. These studies were performed in India,58 Malawi,59 the USA60 and the UK.61 Two studies reported results in HIV-positive patients.59,60

Due to the recognition that non-respiratory TB can have low positive culture rates, studies often base a firm TB diagnosis on histology or culture. A positive histology result is thus not necessarily considered to be inaccurate in the presence of a negative culture. For this reason, there are few studies which consider the sensitivity of histology from biopsy compared to culture alone as the reference standard. Studies merely report the numbers positive on each test. This is not useful for calculating the sensitivity of histology as it is necessary to know the results for each patient on both tests.

These studies were not blinded, mostly because they were retrospective analyses. The majority of specimens used in these studies were lymph nodes and little information is available concerning whether sensitivity and/or specificity may differ when using specimens from other sites.

Although the diagnostic accuracy of individual tests was considered in isolation, in reality test results would not be considered in isolation but would contribute to the overall evidence on which a diagnosis is made.

Diagnosing active non-respiratory TB if culture results are negative

Studies of patients with suspected non-respiratory TB where the results of histology from biopsy or tuberculin skin test were used to support a positive diagnosis in those with a negative culture for TB were searched for.

As with respiratory TB, culture is not a perfect gold standard and may be negative in TB cases for several reasons. In particular in non-respiratory TB, this may be due to pauci-bacillary disease.

No studies were identified in culture-negative populations where the results of histology from biopsy or tuberculin skin tests were used to support a positive diagnosis.

5.2.3. Evidence statements: diagnosing active respiratory TB while awaiting culture results

Sputum microscopy

In a comparison in the USA45 of direct and concentrated specimens, results were analysed for the first three sputum specimens received from patients who were culture-positive for M. tuberculosis and from whom three or more specimens were received. The cumulative proportion of positive smears for each of the three smears for concentrated specimens were 74%, 83% and 91% and this was 57%, 76% and 81% for direct smears. (2)

Sensitivity of smears (all smears, not per patient) using more than or equal to 5 ml of sputum volume in a study in the USA46 was 92%. This was significantly greater than a sensitivity of 72.5% in a previous period when all specimens were processed regardless of volume. In both periods the specificity of acid-fast smear for M. tuberculosis was comparable at 98%. (2)

The rates of smear positivity were calculated for specimens of expectorated sputum, induced sputum and bronchoalveolar lavage (BAL) specimens in a study in the USA.43 Findings of smears of expectorated sputum specimens showed that 55% were culture positive for M. tuberculosis and were AFB smear positive. Smear positivity rates for induced sputum were 38% and for BAL were 26%. When the predictive value was calculated by including only the first smear-positive specimen from each patient the values were 87% for expectorated sputum, 70% for induced sputum and 71% for BAL. (2)

A Turkish study48 compared Ziehl-Neelsen (ZN) and fluorescence microscopy (FM) staining of sputum smears. Where only one specimen was submitted the sensitivities of ZN and FM stains were found to be 61% and 83% respectively. When two were submitted the sensitivities were 66% and 83% and where three or more were submitted sensitivities were 80% and 92%. (3)

In a US study43 of expectorated sputum specimens that were culture positive for TB, 55% of specimens from both patients with and without AIDS (mean 2.4 specimens per patient for both groups) were smear positive. (3)

In a group of non-HIV infected, culture-positive TB patients in the USA,47 57% had positive acid-fast smears compared with 60% of the HIV-infected patients with culture-positive TB (all had at least three specimens tested). Among the TB culture-positive HIV-infected patients, no significant differences were found in the frequency of positive acid-fast sputum smears between groups stratified by CD4 cell counts (in those with a CD4 count of <50, 58% had positive smears, with a CD4 count of 50–200, 60% had positive smears and with a count of >200, 56% had positive smears). (3)

In a USA study,44 70% of all HIV-infected culture-positive TB patients and 71% of all non-HIV infected culture-positive TB patients had at least one positive smear (up to three were performed). The sensitivity for the diagnosis of TB dropped to 55% and 64% respectively when only the first smear was considered. (3)

Chest X-ray

According to X-ray category in a Danish study,49 positive predictive values and sensitivity for TB were 61% and 67% respectively with X-ray changes thought to be due to TB. These values were 20% and 19% with X-ray changes compatible with TB; 14% and 9% with previous TB and radiographically active TB; 2% and 3% with previous TB but not radiographically active TB and 1% and 2% with X-ray changes thought to be due to other disease. None of the patients with normal chest X-rays were culture positive. (1)

In a South African study50 of the diagnostic accuracy of X-ray in children, the results yield a sensitivity of 38.8% and a specificity of 74.4% compared to culture for the diagnosis of pulmonary TB using standard radiographs. (3)

In a group of culture-positive adult AIDS patients a US study51 found 36% of patients had a primary M. tuberculosis pattern, 28% had a post-primary M. tuberculosis pattern, 14% had normal radiographs, 13% had atypical infiltrates, 5% had minimal radiographic changes and 3% had a miliary pattern. Normal chest radiographs were seen for 10 (21%) of 48 patients with less than 200 T-cells per microlitre and one (5%) of 20 patients with more than 200 T-cells per microlitre (p<0.05). (2)

In a US study52 of TB culture-positive adults, 78% of HIV-negative patients’ radiographs were consistent with post-primary pattern TB versus 26% of patients who were HIV positive (p<0.001). Only 11% of 18 significantly immunosuppressed HIV-positive patients (CD4 counts <200) had X-rays consistent with post-primary pattern TB, while all four patients with CD4 counts >200 had typical post-primary pattern chest radiographs (p<0.005). Of the 16 significantly immunosuppressed HIV positive patients the predominant chest X-ray finding was diffuse or multilobar infiltrates without an upper lobe predominance (N=8) followed by normal chest X-ray (N=3). (3)

Gastric washings

In a study of Haitian children54 the sensitivity, specificity and predictive value of positive fluorescence microscopy of gastric washings compared with culture were 58%, 95% and 81% respectively from 536 specimens (median three specimens per patient). Among 49 children with at least one positive fluorescence microscopy of gastric washings, pulmonary TB was bacteriologically confirmed in 85%. Specimens were more frequently positive in far-advanced and miliary disease (82%) than in less severe disease (32%) (p<0.001). (3)

Culture was grown in 16 gastric washings samples in a study of Indian children53 and smears for AFB were positive in only three samples, thus sensitivity was 3/16 or 19% (most children had only one sample taken). (3)

A South African study55 of children with suspected TB found that sensitivity of gastric lavage compared with culture was 39%, specificity was 99%, positive predictive value was 88% and negative predictive value was 90% (based on three gastric lavage samples). Similar results were found for induced sputum specimens, however the yield of culture positive cases from each method was 88% from induced sputum and 66% from gastric lavage. (2)

5.2.4. Evidence statements: diagnosing active respiratory TB if culture results are negative

In South African black male goldmine employees with small lesions in the lung apices on chest X-ray and positive skin tests but negative sputum culture, TB was subsequently diagnosed in 88 (58%) of the 152 men. A diagnosis of TB was made if the smear became positive or the culture yielded M. tuberculosis or if a histological diagnosis was made. Active TB developed in these men from three to 58 months after entering the study, with a mean of 19.8 months.56 (2)

A study performed in Hong Kong of patients with TB diagnosed on the basis of chest X-ray, but with negative culture results, obtained eventual confirmation of active disease requiring treatment in 99 (57%) of 173 patients. During the first 12 months 43% had a confirmed diagnosis. Confirmation of TB was by culture of M. tuberculosis from sputum, or by radiographic or clinical deterioration. There was bacteriological confirmation in 41%. (3)

5.2.5. Evidence statements: diagnosing active non-respiratory TB while awaiting culture results

In patients who presented with lymphadenopathy in one or more extra-inguinal sites in Malawi59 and who did not respond to general antibiotics, it could be calculated that the sensitivity of histology compared to culture was 70%, the specificity was 59%, the positive predictive value was 52% and the negative predictive value was 67%. (2)

In a US study60 of lymph node specimens where the cytology report was compared with culture results, the sensitivity of cytology was calculated to be 72%. (2)

The sensitivity of histology (using a variety of specimens although most frequently lymph nodes) compared with culture in an East London population was 97% with a positive predictive value of 69%.61 (2)

Where culture was the gold standard, an Indian study,58 calculated that in clinically suspected cases of tuberculous lymphadenitis, sensitivity, specificity and positive predictive values for cytology were 78.5%, 73% and 76.7% respectively. (1)

HIV-positive

In a study in Malawi59 in HIV-negative patients with TB lymphadenitis (diagnosed on the basis of a positive culture or histology result), 100% had positive histology results and 83% had positive culture results. These figures were 78% and 56% for those who were HIV positive. Thus the HIV status of the TB lymphadenitis patients suggests a negative influence of HIV infection on the possibility of both histology and culture being indicative of TB (OR 0.10, 95%CI 0 to 1.17, p=0.06). (2)

In a US study60 of lymph node specimens where the cytology report was compared with culture results the sensitivity of cytology in those who were HIV negative was 76% and it was 69% in those who were HIV positive. (2)

5.2.6. From evidence to recommendations

The Chief Medical Officer’s TB Action Plan2 calls for primary and community care staff to be aware of ‘the signs and symptoms of the disease, local TB services and local arrangements for referring patients with suspected TB’. As this guideline is aimed at generalist clinicians as well as those working regularly with people with tuberculosis, recommendations include signs, symptoms and potentially helpful imaging techniques. NICE guidelines generally do not include service guidance (although exceptions have been made elsewhere in this guideline), and so recommendations for local referral are not given.

The GDG were aware of the General Medical Council’s advice62 on gaining consent for testing for ‘serious communicable diseases’, but noted that this advice was reprinted from prior guidance specific to HIV and did not feel that routine clinical practice supported it in TB, and that it was at variance with the Public Health Act.63

Testing for active respiratory TB while awaiting culture results

The yield of positive sputum microscopy is improved by an adequate sputum sample (5 ml or more), concentration of sputum, analysing multiple samples, and by fluorescence microscopy as the screening tool. Smear positive rates are higher for spontaneously induced sputum than for either induced sputum or BAL samples. The positive predictive value of positive sputum microscopy is 92% for spontaneously produced sputum, and 71% for both BAL and induced sputum. There appeared to be little difference in the results between HIV-positive and HIV-negative patients in terms of bacteriological results and sputum smear positivity. Microscopy on gastric washings has some utility in children, but a recent comparative study in children showed a single induced sputum (by hypertonic saline) to be superior to three gastric washings. Gastric washings are less likely to provide useful material in adults, because of acidic inhibition. Chest X-ray changes are less specific in children and HIV-positive individuals, particularly if the CD4 count is under 200 cells/μl.

Testing for active respiratory TB if culture results are negative

The evidence does not assess the adequacy of the respiratory samples sent for culture; a negative culture result can reflect no growth at that time, while a positive result may be obtained later. Chest X-ray appearances consistent with TB were noted to show progression to culture-proven disease in over 50% of subjects in the studies analysed from South Africa and Hong Kong. The decision whether to start TB treatment will be a clinical one based on experience, context and appraisal of all the individual’s results. Further culture samples are sometimes needed after treatment has begun, and will remain viable for a few days, though growth may be slower; the GDG agreed a threshold of one week in this regard.

Interferon-gamma tests may also have a role in ruling out infection with M. tuberculosis; this area is developing rapidly and may need to be updated ahead of the rest of the guideline in 2008.

Testing for active non-respiratory TB while awaiting culture results

Microscopy can be strongly suggestive of TB with certain patterns, and this is often confirmed by a positive culture if material has been sent. Although the data were entirely for peripheral lymph nodes, the GDG thought that this was likely also to apply to other non-respiratory sites.

The decision to biopsy should not be influenced by concerns about sinus formation, as there is no evidence to support this with modern chemotherapy.

Patient preferences are an important consideration in choosing biopsy or needle aspiration.

Posterior–anterior chest X-rays in people with suspected non-respiratory disease are helpful through detecting any coexisting respiratory disease, which will aid or confirm the diagnosis, and be another potential source of bacteriological confirmation. The GDG also agreed a range of other potential tests and imaging techniques.

Testing for active non-respiratory TB if culture results are negative

Although there was no evidence in this area, the GDG noted that continuous enhanced surveillance by the Health Protection Agency (HPA) shows that only some 55% of cases of TB are culture confirmed, and that this is often because no samples have been obtained, with the diagnosis being entirely histological. (However, other reasons include failures in the reporting system and limitations of the matching between Enhanced Tuberculosis Surveillance and MycobNet systems.) To raise the proportion of TB cases diagnosed, particularly at non-respiratory sites, more samples from common TB sites should be sent for TB bacteriology, which requires the education of those sending samples such as general, ENT and orthopaedic surgeons and radiologists performing biopsies.

Interferon-gamma tests may also have a role in ruling out infection with M. tuberculosis; this area is developing rapidly and may need to be updated ahead of the rest of the guideline in 2008.

RECOMMENDATIONS
R2.

To diagnose active respiratory TB:

  • a posterior–anterior chest X-ray should be taken; chest X-ray appearances suggestive of TB should lead to further diagnostic investigation [C(DS)]
  • multiple sputum samples (at least three, with one early morning sample) should be sent for TB microscopy and culture for suspected respiratory TB before starting treatment if possible or, failing that, within seven days of starting [C(DS)]
  • spontaneously produced sputum should be obtained if possible; otherwise induction of sputum or bronchoscopy and lavage should be used [B(DS)]
  • in children unable to expectorate sputum, induction of sputum should be considered if it can be done safely, with gastric washings considered as third line [B(DS)]
  • if there are clinical signs and symptoms consistent with a diagnosis of TB, treatment should be started without waiting for culture results (see section 6.1 for details) [D(GPP)]
  • the standard recommended regimen should be continued in patients whose subsequent culture results are negative [D(GPP)]
  • samples should be sent for TB culture from autopsy samples if respiratory TB is a possibility. [D(GPP)]
R3.

To diagnose active non-respiratory TB:

  • advantages and disadvantages of both biopsy and needle aspiration should be discussed with the patient, with the aim of obtaining adequate material for diagnosis [B(DS)]
  • if non-respiratory TB is a possibility, part or all of any of the following samples should be placed in a dry pot (and not all placed in formalin) and sent for TB culture: [D(GPP)]
    • – lymph node biopsy
    • – pus aspirated from lymph nodes
    • – pleural biopsy
    • – any surgical sample sent for routine culture
    • – any radiological sample sent for routine culture
    • histology sample
    • – aspiration sample
    • – autopsy sample
  • microbiology staff should routinely perform TB culture on the above samples (even if it is not requested) [D(GPP)]
  • the appropriate treatment regimen should be started without waiting for culture results if the histology and clinical picture are consistent with a diagnosis of TB (see chapters 6 and 7) [C(DS)]
  • all patients with non-respiratory TB should have a chest X-ray to exclude or confirm coexisting respiratory TB; in addition, tests as described in Table 7 should be considered [D(GPP)]
  • the appropriate drug regimen (see chapters 6, 7 and 9) should be continued even if subsequent culture results are negative. [D(GPP)]

Cross-referring:

For details of rapid diagnostic tests, see sections 5.3 and 5.4.

For people with active TB, see treatment under chapters 6, 7 and 9.

For details of contact tracing, see section 12.2.

For details of notification and enhanced surveillance, see chapter 14.

5.3. Rapid diagnostic tests: molecular methods

5.3.1. Clinical introduction

Molecular probes for diagnosis

A number of methods have been developed which target and amplify specific regions of mycobacterial DNA, thus allowing a rapid result. However, such tests can result in false negative and false positive findings. Although rare, false positive results may occur due to contamination of the sample with environmental mycobacteria causing non-specific binding to the probe. More commonly, false negative results may occur due to low organism numbers or, in some sample types, for example CSF, to the presence of inhibitors. The specificity and sensitivity of the tests has been compared with culture proven disease. However, since 20–30% of pulmonary cases, and a higher proportion of non-pulmonary cases are not culture proven, the performance of molecular tests in these settings is difficult to assess.

Molecular probes for species confirmation

Species identification may sometimes be possible directly from the specimen using the techniques referred to above. Most usually, this will be possible only for M. tuberculosis complex organisms (M. tuberculosis, M. bovis, M. africanum). However, these methods may allow early differentiation between these organisms and environmental mycobacteria. These tests are most effective when applied to samples in which mycobacteria have been detected microscopically. Their use is currently recommended, to confirm true tuberculosis (ie transmissible disease) before a large contact tracing exercise, for example in a school or hospital, is carried out.6

When a sample yields a positive culture, rapid identification of several commonly encountered species may be possible. This may be done by the application of an expanded range of DNA amplification-based assays or by the use of non-amplified hybridisation probes. Both of these approaches are effective since the high numbers of organisms present in a positive culture overcome the problems associated with low bacterial counts and inhibition in the primary sample. The Mycobacterium Reference Service of the HPA now routinely confirms to clinicians whether a positive culture received is from the M. tuberculosis complex or not.

Molecular probes for rifampicin resistance

The incidence of multi-drug resistant strains of M. tuberculosis (MDR TB) in the UK is low (~1%) (see Appendix E). However, in some areas of the country and in some population groups the incidence is much higher. Whilst it should be noted that mono-resistance to rifampicin is found in approximately 5% of rifampicin-resistant strains, a high proportion of rifampicin resistance is associated with concurrent resistance to isoniazid (~95%). Thus the detection of resistance to rifampicin can be used as a marker for MDR TB with a high level of accuracy.

Rifampicin resistance is commonly due to one or more of several possible mutations of the rpoB gene and these can be detected using a PCR-based technique. A positive result from such a test should lead to the implementation of infection control measures and drug treatment for MDR TB until the results of standard drug susceptibility tests are available. Risk factors for MDR TB, which should lead to such tests for rifampicin resistance, are listed in section 9.1. Clinicians should be aware that there is a small (<5%) false negative rate for these tests as a few mutations conferring rifampicin resistance are not at the rpoB gene tested for.64,65

Molecular typing of M. tuberculosis isolates

In the past the typing of M. tuberculosis strains has been principally to detect previous events. This was largely due to the comparatively slow techniques available (for example, restriction fragment length polymorphisms). Newer methods based on the detection of variable numbers of tandem repeat sequences within the M. tuberculosis genome (variable number of tandem repeats (VNTR)/mycobacterial interspersed repetitive unit (MIRU) typing) are amenable to automation. As a result rapid, high-throughput typing systems have become available. These systems also have the advantage of digitised data which allow much easier computerised storage and analysis than previous typing methods. If this rapidity of method is used to type strains as they are isolated, then potential links between patients may be detected early enough to interrupt the disease transmission process. Thus an epidemiological tool may make an impact on diagnosis and transmission.

5.3.2. Methodological introduction

In consideration of the use of molecular methods for rapid diagnosis of TB, the review being developed by the NHS Heath Technology Assessment Programme66 has been adopted. This aims to conduct a systematic review of the effectiveness of available diagnostic tests to identify mycobacteria. The review is not yet published.

The draft review of nucleic acid amplification tests (NAAT) found 163 studies which compared NAAT with a reference standard. There were 105 comparisons in respiratory specimens and 67 in non-respiratory specimens. In these studies 77 of the tests used were commercially produced (the amplified Mycobacterium tuberculosis direct (AMTD) test, the Amplicor, the Ligase Chain Reaction and Ampicis Myco B) and 86 were produced in-house (insertion element IS6110 or other targets).

Methodological issues concern the complexity of pooling data from diagnostic studies in particular due to variation in diagnostic thresholds. Furthermore, studies report pairs of related summary statistics (sensitivity and specificity) rather than a single statistic, requiring alternative statistical methods for pooling results. This review presents diagnostic odds ratios (DOR) in addition to sensitivity and specificity data. This is a single summary of diagnostic performance which although not easy to apply in clinical practice (it describes the ratio of the odds of a positive test result in a patient with disease compared to a patient without disease) is convenient to use when combining studies as it is often fairly constant regardless of diagnostic threshold. The DOR can be calculated from sensitivity and specificity data and where a test provides no diagnostic evidence the DOR is 1. It has been suggested67 that a DOR of 25 or more in a test may provide convincing diagnostic evidence.

5.3.3. Evidence statements

The health technology appraisal (HTA) on rapid diagnostic tests66 is not yet published. The GDG considered interim results, reporting the DOR statistic calculated by comparing NAAT vs. a reference standard. All evidence is graded at level 2.

5.3.4. From evidence to recommendations

Molecular probes for diagnosis

The HTA of rapid tests showed that their sensitivity was equivalent to culture in microscopy negative pulmonary samples, but there was an increased false negative rate in non-respiratory samples, particularly in pleural fluid and CSF. Significant false negative rates in these settings limit their utility, and could lead to failure to diagnose and treat TB.

Molecular probes for species confirmation

The GDG did not look into the HTA’s interim results for molecular probes, but noted their role in rapid confirmation. They were not felt to be more reliable or useful than culture confirmation, and use was therefore limited to occasions when a rapid decision is needed on treatment or infection control measures. A further role was in preventing large scale contact tracing exercises from starting unnecessarily.

Molecular tests are less feasible on poorer samples, and the recommendations given below advise on their use on biopsy material.

Molecular probes for rifampicin resistance

Again, the GDG recognised the advantages of rapid results for drug resistance, but noted that MDR TB risk factors should be used to determine infection control measures at the earliest opportunity.

Molecular typing of M. tuberculosis isolates

Although this has not been subject to formal HTA appraisal, these methods have been considered by the HPA and a unified strategy using a 15 locus VNTR/MIRU system agreed. Such a strategy was recommended in the TB Action Plan.2

RECOMMENDATIONS
R4.

Rapid diagnostic tests for M. tuberculosis complex (M. tuberculosis, M. bovis, M. africanum) on primary specimens should be used only if: [D(GPP)]

  • rapid confirmation of a TB diagnosis in a sputum smear-positive person would alter their care, or
  • before conducting a large contact-tracing initiative.
R5.

Clinicians should still consider a diagnosis of non-respiratory TB if rapid diagnostic tests are negative, for example in pleural fluid, CSF and urine. [B(DS)]

R6.

Clinical signs and other laboratory findings consistent with TB meningitis should lead to treatment (see section 7.1), even if a rapid diagnostic test is negative, because the potential consequences for the patient are severe. [D(GPP)]

R7.

Before conducting a large contact-tracing initiative (for example, in a school or hospital), the species of mycobacterium should be confirmed to be M. tuberculosis complex by rapid diagnostic tests on microscopy- or culture-positive material. Clinical judgement should be used if tests are inconclusive or delayed. [D(GPP)]

R8.

If a risk assessment suggests a patient has MDR TB (see section 7.1): [D(GPP)]

  • rapid diagnostic tests should be conducted for rifampicin resistance
  • infection control measures and treatment for MDR TB should be started as described in chapter 9, pending the result of the tests.
R9.

Rapid diagnostic tests for M. tuberculosis complex identification should be conducted on biopsy material only if: [D(GPP)]

  • all the sample has been inappropriately placed in formalin, and
  • AFB are visible on microscopy.

Cross-referring:

For details of managing drug-susceptible TB, see chapters 6 and 7.

For details of managing drug-resistant TB, see chapter 9.

5.4. Rapid diagnostic tests: automated liquid culture

5.4.1. Clinical introduction

Clinicians have been advised to obtain culture confirmation of tuberculosis whenever possible.68 This not only confirms the diagnosis, but crucially also obtains material for drug susceptibility testing, which is important because of the current levels of drug resistance in England and Wales. The finding of isoniazid resistance (currently 6% of isolates) requires modification of treatment (see section 9.4), and that of MDR TB (currently about 1% of isolates) different infection control procedures (see section 9.3) and individualised treatment regimens based on the drug susceptibility data.

Until recently, culture for mycobacteria was done mainly on solid media, the Lowenstein-Jensen slope, or in broth media. These methods were slow, with cultures from microscopy positive material taking from 2–4 weeks, and for microscopy negative material 4–8 weeks. More recently rapid culture methods have been developed, with the potential advantages of more rapid growth and hence earlier drug susceptibility data, and also possibly increased sensitivity.

The national TB Action Plan has as one of its aims the use of rapid culture methods for diagnosis of all cases of tuberculosis.2

5.4.2. Methodological introduction

The reduced turnaround time of automated liquid culture in comparison with solid media is uncontested. In addition to time to detection of mycobacteria, study outcomes in comparisons between solid and liquid media also report increases recovery rates for mycobacteria.66 Sensitivity and or specificity cannot be reported in these studies as there is no reference standard.

There were no studies identified which directly addressed the issue of when (ie in what circumstances) automated liquid culture methods for the diagnosis of TB are most useful.

The HTA on rapid diagnostic techniques66 is not yet published. The GDG considered interim findings on liquid culture techniques.

5.4.3. From evidence to recommendations

Given the evidence base and the self-evident speed of automated liquid culture, the GDG recommended their universal use.

Liquid culture methods require batches of samples to be processed. Their use becomes more costly per test if fewer samples are processed at any one time by a laboratory. The batching of samples sent to regional laboratories may not reflect future service organisation as this technology becomes more widely used over the lifetime of this guideline, but the recommendations allude to the effect of throughput on efficiency, quality control and cost-effectiveness. The NICE guideline, in the absence of clinical evidence, is unable to recommend service configurations to address this, though the GDG considered a ‘hub and spoke’ arrangement of regional laboratories.

RECOMMENDATIONS

R10.

Clinical samples should ideally be sent for culture by automated liquid methods, bearing in mind that laboratories need a certain level of throughput to maintain quality control. [D(GPP)]

Copyright © 2006, Royal College of Physicians of London.

All rights reserved. No part of this publication may be reproduced in any form (including photocopying or storing it in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright owner. Applications for the copyright owner’s written permission to reproduce any part of this publication should be addressed to the publisher.

Bookshelf ID: NBK45813

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