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Screening Donated Blood for Transfusion-Transmissible Infections: Recommendations. Geneva: World Health Organization; 2009.

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Screening Donated Blood for Transfusion-Transmissible Infections: Recommendations.

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4Screening for transfusion-transmissible infections

4.1. TRANSFUSION-TRANSMISSIBLE INFECTIONS

The microbial agents of importance to blood transfusion services are those that are transmissible by blood transfusion and can cause morbidity and mortality in recipients. In order to be transmissible by blood, the infectious agent or infection usually has the following characteristics:

  • Presence in the blood for long periods, sometimes in high titres
  • Stability in blood stored at 4°C or lower
  • Long incubation period before the appearance of clinical signs
  • Asymptomatic phase or only mild symptoms in the blood donor, hence not identifiable during the blood donor selection process (25).

Infections that consistently meet these criteria include those described in Section 4.2.

As large volumes of blood or blood components are given to patients during transfusion therapy, even a blood unit with a low viral load may cause infection in the recipient. It is imperative that blood transfusion services have effective screening systems to detect, segregate and remove reactive blood donations and all components derived from these donations from the quarantined useable stock. Only non-reactive blood and blood components should be released for clinical or manufacturing use.

The various markers of infection appear at different times after infection. Each TTI has one or more window periods, ranging from a few days to months, depending on the infectious agent, the screening marker used and the screening technology employed. During this period, the particular screening marker is not yet detectable in a recently infected individual, even though the individual may be infectious. Nucleic acid, as part of the native infectious agent itself, is the first detectable target to appear, followed within a few days by antigen, and subsequently by antibody as the immune response develops.

One or a combination of markers of infection can be used to detect a particular infection during the screening process. Various assay systems developed for blood screening detect:

  • Antibodies that indicate an immune response to the infectious agent
  • Antigens that are produced by the infectious agent and indicate the presence of that agent
  • Nucleic acid (RNA/DNA) of the infectious agent.

In non-endemic countries, where the blood donor population includes travellers to or migrants from endemic areas, alternative strategies may be required, based on selective blood donor deferral and/or screening tests, if suitable assays are available. Similarly, some infections, such as human cytomegalovirus (CMV), present a risk to certain recipient groups only. In this situation, the selective screening of donations for these specific recipients is normally adopted.

GENERAL RECOMMENDATIONS

To minimize the risk of the transmission of infection through the route of transfusion:

  1. All whole blood and apheresis donations should be screened for evidence of the presence of infection prior to the release of blood and blood components for clinical or manufacturing use.
  2. Screening of all blood donations should be mandatory for the following infections and using the following markers:
    • HIV-1 and HIV-2: screening for either a combination of HIV antigen-antibody or HIV antibodies
    • Hepatitis B: screening for hepatitis B surface antigen (HBsAg)
    • Hepatitis C: screening for either a combination of HCV antigen-antibody or HCV antibodies
    • Syphilis (Treponema pallidum): screening for specific treponemal antibodies.
  3. Screening of donations for other infections, such as those causing malaria or Chagas disease, should be based on local epidemiological evidence.
  4. Screening should be performed using highly sensitive and specific assays that have been specifically evaluated and validated for blood screening.
  5. Quality-assured screening of all donations using serology should be in place before additional technologies such as nucleic acid testing are considered.
  6. Only blood and blood components from donations that are non-reactive in all screening tests for all markers should be released for clinical or manufacturing use.
  7. All screen reactive units should be clearly marked, removed from the quarantined stock and stored separately and securely until they are disposed of safely or kept for quality assurance or research purposes, in accordance with national policies.

4.2. TRANSFUSION-TRANSMISSIBLE INFECTIOUS AGENTS FOR WHICH UNIVERSAL SCREENING OF ALL DONATION IN ALL COUNTRIES IS RECOMMENDED

Screening for the following four infections that are transmissible by transfusion is recommended as mandatory for the provision of a safe blood supply. These infections can cause chronic disease with possible serious consequences and present the greatest infection risk to recipients of transfusion:

  • human immunodeficiency virus (HIV)
  • Hepatitis B virus (HBV)
  • Hepatitis C virus (HCV)
  • Treponema pallidum (syphilis).

Importantly, the risks of infection can be virtually eliminated if the screening of blood donations is performed in a quality-focused way. All efforts should be made to implement universal screening for these four infections by countries in which it is not currently fully in place.

All blood donations should be screened for at least one suitable serological marker for each of these four infections. Screening for additional markers for these infections and for other transfusion-transmissible infectious agents could then be considered, depending on the residual risk, logistics and level of resources available.

4.2.1. Human immunodeficiency virus

Agent

The human immunodeficiency virus (HIV) is a retrovirus, an enveloped RNA virus, which is transmissible by the parenteral route. It is found in blood and other body fluids. Once in the bloodstream, the virus primarily infects and replicates in lymphocytes. The viral nucleic acid persists by integrating into the host cell DNA.

A number of different groups and subtypes (clades) have been identified with some significant antigenic differences; HIV-1 and HIV-2 are the two major distinct virus types and there is significant cross-reactivity between them. HIV-1 is now endemic in many parts of the world, although its incidence and prevalence is low in some regions. HIV-1 group M is responsible for more than 99% of the infections worldwide, whereas the prevalence of HIV-2 is mainly restricted to countries in West Africa and India. Additionally, a few infections with HIV group O and group N have been observed in Africa. The appearance of antibody marks the onset and persistence of infection, but not immunity.

Transmissibility

As HIV can be present in the bloodstream in high concentrations and is stable at the temperatures at which blood and individual blood components are stored, the virus may be present in any donated blood from an HIV-infected individual. Infectivity estimates for the transfusion of infected blood products are much higher (around 95%) than for other modes of HIV transmission owing to the much larger viral dose per exposure than for other routes (26).

Screening

The methods used to identify the presence of HIV employ the following screening targets:

  • Serological markers:

    anti-HIV-1, including group O, + anti-HIV-2

    HIV p24 antigen (p24 Ag)

  • Viral nucleic acid: HIV RNA.

The assay should be capable of detecting subtypes specific to the country or region.

Screening donations for both antibody and antigen will identify the vast majority of donations from infected donors (27).

anti-HIV-1 + anti HIV-2 and p24 antigen

All screening strategies should employ, at minimum, the detection of antibody because the identification of specific antibody is still the most reliable screening method. They should preferably also employ the detection of antigen. Antibody may be detected approximately three weeks after infection and approximately six days after antigen is first detected (28). HIV p24 antigen may appear from 3 to 10 days after viral RNA (29), and its detection can further reduce the serological window period by 3 to 7 days before antibody detection.

Screening for anti-HIV has been the basis for blood screening since the mid-1980s and HIV serology is therefore well understood. Although there is cross-reactivity between the main virus types (HIV-1 and HIV-2), it is not sufficient to rely on an HIV-1 specific assay to detect all cases of HIV-2. Since the early 1990s, anti-HIV assays have included specific antigens for both HIV-1 and HIV-2. However, the use of antibody-only assays has been superseded by the use of combination HIV antigen and antibody assays (combined HIV p24 Ag and anti-HIV-1 + anti-HIV-2), wherever possible. These provide an enhanced level of sensitivity in early infection over antibody-only assays by reducing the serological window period (30).

HIV RNA

Viral RNA can be detected approximately 7 to 11 days after infection: i.e. when the results of HIV antigen-antibody assays are negative, but HIV RNA detection is positive (28). The detection of HIV RNA can reduce the risk of HIV being transmitted through the transfusion of infected blood donated during the serological window period of antigen and antibody assays.

RECOMMENDATIONS

To minimize the risk of HIV infection through the route of transfusion:

  1. Screening should be performed using a highly sensitive and specific anti-HIV-1 + anti-HIV-2 immunoassay or HIV combination antigen-antibody immunoassay (EIA/CLIA). The assay should be capable of detecting subtypes specific to the country or region.
  2. Screening using a highly sensitive and specific anti-HIV-1 + anti-HIV-2 rapid assay may be performed in laboratories with small throughput, in remote areas or emergency situations.

4.2.2. Hepatitis B virus

Agent

Hepatitis B virus (HBV) is a member of the hepadnavirus group and is an enveloped DNA virus. HBV is transmissible by the parenteral route and may be found in blood and other body fluids. Once in the bloodstream, the virus travels to the liver where it replicates in hepatocytes.

HBV is endemic globally and hyper-endemic in parts of the world. It is difficult to determine the total number of cases of transfusion-transmitted HBV globally.

Transmissibility

While HBV is present in the bloodstream, the levels of the virus itself are variable. In recently infected individuals, viral DNA is normally present, although not always at high levels. Chronically infected individuals may either be infectious (viral DNA present) or non-infectious (viral DNA absent) and viraemia would generally be expected to be very low or absent entirely. Screening for hepatitis B surface antigen (HBsAg) indicates infection with HBV, but does not in itself distinguish between recent and chronic infections.

The distinction between acute and chronic infection is not relevant to blood screening; all HBsAg positive donations should be considered to be at high risk of transmitting HBV and should not be released for transfusion. Additionally, some studies indicate that even when HBsAg is negative, some individuals may have low levels of detectable viral DNA which will be transmitted by blood and may cause infection in the recipient (31-32).

The use of unscreened HBV-infected blood and blood products will result in the transmission of HBV in the vast majority of cases. In general, the earlier in life that HBV is acquired, the more likely the individual is to develop chronic infection which then has a higher probability of progressing to cirrhosis and hepatocellular carcinoma.

Screening

The serology of HBV is complex. A number of different serological markers develop during the course of infection, including hepatitis B surface antigen (HBsAg) and hepatitis B core antibody (anti-HBc). In addition, HBV DNA can be detected in the majority of cases, although in HBsAg negative phases of infection the DNA levels are generally relatively low and the viraemia may be transient.

The methods used to identify the presence of HBV employ the following screening targets:

  • Serological markers:

    Hepatitis B surface antigen

    Hepatitis B core antibody, in some situations

  • Viral nucleic acid: HBV DNA.
Hepatitis B surface antigen

Hepatitis B surface antigen is the prime marker used in blood screening programmes. It normally appears within three weeks after the first appearance of HBV DNA and levels rise rapidly (31).

It can thus be detected easily by most of the highly sensitive HBsAg assays available. The presence of HBsAg may indicate current or chronic infection and thus potential infectivity. Most blood transfusion services screen donated blood for HBsAg using sensitive immunoassays. Particle agglutination assays are still available and used in some countries, although they are less sensitive than immunoassays or even simple/rapid assays.

Hepatitis B core antibody

Antibody to hepatitis B core antigen is produced later in acute infection, after the appearance of HBsAg, and marks the start of the immune response to HBV infection. In general, anti-HBc persists for life, irrespective of whether the infection resolves or progresses to chronicity. In the vast majority of cases of hepatitis B, the detection of anti-HBc has limited value as HBsAg is already present. In some cases, however, during the resolution of the infection, HBsAg may decline to below detectable levels. Although anti-HBs usually then appears relatively rapidly, there may be a short period of time prior to its appearance when anti-HBc is the only detectable circulating serological marker of infection, even though the individual may still have low viraemia and would thus be potentially infectious.

If anti-HBc screening is introduced for routine use, it would be necessary to distinguish between individuals who are anti-HBc reactive because of previous, resolved, natural HBV infection, and are thus non-infectious, from those who have unresolved HBV infection and are thus potentially infectious. In a population with a high prevalence of infection, the number of blood donors with evidence of natural, resolved infection is likely to be significant, resulting in the potentially unnecessary discard of many blood donations. As the presence of anti-HBs is protective, anti-HBs testing of all anti-HBc reactive donations would therefore be required to distinguish between infectious and non-infectious individuals. In general, a level of anti-HBs at 100 mIU/mL is usually accepted as the minimum protective level in the context of blood screening; donations that are HBsAg negative, anti-HBc reactive with anti-HBs levels of 100 mIU/mL or more are generally considered to be safe and acceptable for release for clinical or manufacturing use.

Another important consideration is that anti-HBc assays often demonstrate a high level of non-specificity (33). This, together with the problems associated with the confirmation of anti-HBc reactivity, often results in a situation where anti-HBc reactivity is identified in the absence of any other markers of HBV infection and where the majority of this reactivity is actually non-specific and does not reflect HBV infection. Thus, although anti-HBc screening may have advantages in some situations, the problems associated with the performance of anti-HBc assays and the complexity of dealing with HBV immune individuals may outweigh any potential benefits.

Alanine aminotransferase

Testing for raised liver alanine aminotransferase (ALT) levels was originally introduced in some countries prior to the identification of hepatitis C and the introduction of HCV screening in an attempt to reduce the incidence of what was then called post-transfusion non-A, non-B hepatitis (PTNANBH) (34). ALT is an enzyme found predominantly in the liver. It circulates naturally at low levels in the bloodstream, but is released in high quantities as a result of liver damage; this is often, but not exclusively, due to viral infection.

ALT is a non-specific marker of infection. With the advent of HCV screening, screening for raised ALT levels provides no identifiable benefit in terms of improving blood safety (35).

Hepatitis B virus DNA

The detection of HBV DNA further reduces the risk of HBV transmission through the transfusion of infected blood donated during the acute window period: i.e. when the results of HBsAg assays are negative, but HBV DNA is positive (36). Low levels of HBV DNA have also been detected in the blood of individuals after the resolution of acute HBV infection and the disappearance of HBsAg or in so-called chronic occult HBV infection (3132).

RECOMMENDATIONS

To minimize the risk of HBV infection through the route of transfusion:

  1. Screening should be performed using a highly sensitive and specific HBsAg immunoassay (EIA/CLIA).
  2. Screening using a highly sensitive and specific HBsAg rapid assay or particle agglutination assay may be performed in laboratories with small throughput, in remote areas or in emergency situations.
  3. Screening for anti-HBc is not recommended as a routine. Countries should determine the need for anti-HBc screening based on the prevalence and incidence of HBV infection.
  4. Screening for ALT is not recommended.

4.2.3. Hepatitis C virus

Agent

Hepatitis C virus (HCV) is a member of the flavivirus group and is an enveloped RNA virus. It is transmissible by the parenteral route and may be found in blood and other body fluids. Once in the bloodstream, the virus travels to the liver where it replicates in hepatocytes, resulting in a similar picture to that seen with HBV infection. Seroreversion has been seen in numbers of individuals who have resolved their infections. The loss of circulating antibody may leave no readily detectable evidence of previous infection (37).

HCV is endemic in many parts of the world, although in some regions its incidence and prevalence may be low. Several genotypes are identified and are associated with different geographical distributions and some differences in antigenicity and clinical features, including response to treatment with interferon alpha (IFN-α).

Transmissibility

While HCV is present in the bloodstream, the levels of the virus itself are variable. In recently infected individuals, virus is normally present. However, only around 70% of chronically infected individuals are viraemic and the length of time that viraemia persists is not fully understood. Nonetheless, it is expected that most HCV infected donations would contain virus and thus be infectious.

Screening for both HCV antigen and antibody does not in itself distinguish between recent and chronic infection. The distinction is, however, not relevant to the screening of blood for transfusion and all HCV antigen-antibody reactive donations should be considered to be at high risk of transmission of HCV and should not be used for clinical or manufacturing use.

Screening

The methods used to identify the presence of HCV employ the following screening targets:

  • Serological markers:

    HCV antibody

    HCV antigen

  • Viral nucleic acid: HCV RNA.
HCV antibody and antigen

HCV antibody becomes detectable approximately 30 to 60 days after infection. Viral antigen normally appears between 0 and 20 days after viral RNA first appears. Antibody is generated and can be detected between 10 and 40 days after antigen is first detected.

The serology of HCV is still not fully understood. Serological screening has been highly effective in significantly reducing the transmission of HCV through the route of transfusion. Until recently, anti-HCV has been the prime serological marker for blood screening programmes. However, HCV antigen can be detected in the peripheral blood earlier than antibody in the course of early infection. HCV antigen assays, both antigen only and combined antigen-antibody, have been commercially available for a number of years. These have been introduced in some countries to improve the overall effectiveness of serological HCV screening (38).

Hepatitis C virus RNA

Viral RNA is normally detectable within a few weeks of infection and persists for 6–8 weeks prior to antibody seroconversion (28). The detection of HCV RNA may further reduce the risk of HCV transmission through the transfusion of infected blood donated during the window period of antigen and antibody assays: i.e. when the results of HCV antigen-antibody assays are negative, but HCV RNA is positive (28). However, any benefit is dependent upon HCV incidence and the actual number of donations that may be collected in the window period (38).

RECOMMENDATIONS

To minimize the risk of HCV infection through the route of transfusion:

  1. Screening should be performed using a highly sensitive and specific HCV antibody immunoassay or a combination HCV antigen-antibody immunoassay (EIA/CLIA). The assay should be capable of detecting genotypes specific to the country or region.
  2. Screening using a highly sensitive and specific HCV antibody rapid assay may be performed in laboratories with small throughput, in remote areas or emergency situations.

4.2.4. Syphilis

Agent

Syphilis is caused by the bacterium Treponema pallidum pallidum. It is transmissible by the parenteral route and may be found in blood and other body fluids. Once in the bloodstream, the bacteria spread throughout the body. A primary lesion, chancre, usually occurs about three weeks after exposure, although the duration may be shorter in cases of transfusion-transmitted infection where the organism enters the bloodstream directly. Syphilis is endemic in many parts of the world.

Transmissibility

While T. pallidum may be found in the bloodstream, levels are variable, even in acute primary syphilis, and the bacteraemia is often short-lived. In addition, the treponemes are relatively fragile, in particular being heat-sensitive; storage below +20°C for more than 72 hours results in irreparable damage to the organism such that it is no longer infectious. Thus, although clearly potentially infectious, the risk of transmission through the transfusion of blood and blood components stored below +20°C is very low.

Blood components stored at higher temperatures (above +20°C), such as platelet concentrates, or those not stored at lower temperatures for any length of time, such as blood collected and used within 48 hours, present a significantly higher risk of transmitting syphilis. Thus, although the risk of transmission of syphilis from unscreened donations is variable, the screening test is nonetheless considered essential as most blood transfusion services provide some blood components that are either stored above +20°C or are not stored below +20°C for sufficient time to kill any organisms present.

Screening

The methods used to identify the presence of syphilis employ the following screening targets:

  • Non-specific, non-treponemal markers: antibody to lipoidal antigen (reagin)
  • Specific treponemal antibodies.

Treponemal serology is relatively complex with different profiles seen at different stages of infection and depending on whether treatment has been given. Infection with the four major types of pathogenic treponemes cannot be distinguished by serological screening because the major immunodominant epitopes are so similar that the antibodies produced are detected by any specific antibody assay for syphilis.

In general, syphilis assays can be divided into specific and non-specific assays; their use depends on whether the purpose of testing is screening or diagnostic testing.

Specific assays

Specific assays commonly used for blood screening are Treponema pallidum haemagglutination assays (TPHA) and enzyme immunoassays (EIAs). These detect specific treponemal antibodies and thus identify donations from anyone who has ever been infected with syphilis, whether recently or long in the past, and whether treated or not.

Non-specific assays

Non-specific assays such as Venereal Diseases Research Laboratory (VDRL) and rapid plasma reagin (RPR) tests identify those individuals who may have been more recently infected. They detect antibodies to cardiolipin or lipoidal antigen (reagin); the plasma levels of these antibodies rise significantly in active infection due to the cellular damage. The use of non-specific assays is of most value in diagnostic testing where it can be used to identify recently infected individuals.

When the incidence and prevalence of syphilis in the blood donor population are high and cannot be reduced through donor selection strategies, it may be necessary to consider screening using a non-treponemal assay (e.g. VDRL or RPR) to identify only the highest-risk donors – those with evidence of recent infections. For routine screening, however, this strategy carries a high risk of false negative results as the sensitivity of these assays is lower than specific assays and the test results may not always be positive, even when the infection is recent.

RECOMMENDATIONS

To minimize the risk of syphilis infection through the route of transfusion:

  1. Screening should be performed using a highly sensitive and specific test for treponemal antibodies: either TPHA or enzyme immunoassay.
  2. In populations where there is a high incidence of syphilis, screening should be performed using a non-treponemal assay: VDRL or RPR.

Table 1Summary of screening markers, assays and recommendations for the four mandatory transfusion-transmissible infections

VirusScreening marker*AssayRecommendationComments
HIVAnti-HIV (1,2,0)Immunoassay:
  • EIA
  • CLIA
Recommended
  • Essential for effective HIV screening; screening for HIV antibody is recommended as minimum standard for blood safety
  • Currently the most efficacious assays are combination antigen-antibody assays
  • Specific detection of antibodies to both HIV-1 and HIV-2 is essential
Anti-HIV (1,2,0)Immunoassay:
  • Rapid
  • Particle agglutination
May be used in special situations
HIV p24 antigenImmunoassay:
  • EIA
  • CLIA
Recommended only if part of combination antigen-antibody assay
  • First serological marker of HIV infection
  • A valuable target for donation screening although viral antigen is neutralized by antibody
  • Screening for HIV antigen only is not appropriate as levels fall as specific antibody levels rise
  • HIV antigen may be detected at the same time or very soon after first detection of HIV RNA
  • Currently the most sensitive HIV serological assays combine detection of both antigen (p24 antigen) and antibody (anti-HIV-1 and -2). These assays are considered to be the most effective for the serological screening of donations
HIV RNANucleic acid amplification technologyEvaluate increased safety vs costs and logistics
  • First circulating marker of HIV infection but the window between the detection of HIV RNA and HIV p24 antigen may be short
  • Screening for HIV RNA has been implemented in a number of countries
  • Value of RNA screening is related to serological screening performed and incidence of infection in donors
Hepatitis BHBsAgImmunoassay:
  • EIA
  • CLIA
Recommended
  • First serological marker of HBV infection
  • Significant quantities of HBsAg produced and released into circulation, the majority not associated with viral nucleic acid
  • Essential for effective HBV screening; screening for HBsAg recommended as minimum standard for blood safety
Immunoassay
  • Rapid
  • Particle agglutination
May be used in special situations
Anti-HBcImmunoassay:
  • EIA
  • CLIA
Not recommended, especially in countries with high HBV prevalence
  • Used as an additional marker in some countries to identify resolving infections when HBsAg has declined below detectable levels but HBV DNA may still be present.
  • May be the only circulating marker of infection at that point
  • Assays may lack specificity and specific confirmation is not available
  • Anti-HBs levels should be determined for all anti-HBc reactive donations to identify resolved infections
  • Accepted in many countries that donations that are both anti-HBc reactive and have an anti-HBs level >100 mlU/mL are suitable for clinical use
Alanine aminotransferaseBiochemical assayNot recommended
HBV DNANucleic acid amplification technologyEvaluate increased safety vs costs and logistics
  • First circulating marker of HBV infection, but with limited usefulness in blood screening unless individual donation testing is performed
  • Virus is generally low-titre and the window between the detection of HBV DNA and HBsAg is generally very short
Hepatitis CAnti-HCVImmunoassay:
  • EIA
  • CLIA
Recommended
  • Currently the most commonly used serological marker of HCV infection
  • Appears in response to infection, but window period from first appearance of viral RNA may be relatively long
  • Screening for HCV antibody recommended as minimum standard for blood safety
RapidMay be used in special situations
HCV antigenImmunoassay:
  • EIA
  • CLIA
Recommended only if part of combination antigen-antibody assay
  • First serological marker of HCV infection
  • A valuable target for donation screening although viral antigen is neutralized by antibody
  • Screening for HCV antigen only is not appropriate as levels fall as specific antibody levels rise
  • HCV antigen may be detected at the same time or very soon after first detection of HCV RNA
  • Very limited assay availability
  • The most sensitive HCV serological assays combine detection of both antigen and antibody. These assays are considered to be the most effective for the serological screening of donations although currently only a limited number of assays are commercially available
HCV RNANucleic acid amplification technologyEvaluate increased safety versus costs and logistics
  • First circulating marker of HCV infection but the window between the detection of HCV RNA and HCV antigen may be short
  • Screening for HCV RNA has been implemented in a number of countries, primarily for the safety of plasma for fractionation
  • Value of RNA screening is related to serological screening performed and incidence of infection in donors
SyphilisAntibody to Treponema pallidum
  • Particle agglutination (TPHA)
  • Immunoassay (EIA)
Recommended
  • First specific serological marker of syphilis infection
  • Essential for effective syphilis screening; screening for specific treponemal antibody recommended as minimum standard for blood safety
Antibody to lipoidal antigen (reagin)
  • VDRL
  • RPR
Consider when high incidence of syphilis
  • First serological marker of syphilis infection
  • Do not detect syphilis specific antibody
  • Lack sensitivity and specificity

Note

*

Markers of infection that are potential screening targets

4.3. TRANSFUSION-TRANSMISSIBLE INFECTIONS FOR WHICH UNIVERSAL SCREENING IS RECOMMENDED IN SOME COUNTRIES OR FOR WHICH SELECTIVE SCREENING IS RECOMMENDED

Infections such as malaria, Chagas disease and the human T-cell lymphotropic viruses I/II (HTLV) may present a greater risk in certain regions and countries, even though there is not the same level of risk across the world. Each country should assess whether any bloodborne infections in addition to HIV, HBV, HCV and syphilis also pose a significant threat to the safety of the blood supply owing to their biology, incidence and/or prevalence in the general population and the subsequent risk of the presence of this infection in blood donors:

  • In endemic areas, specific risks include the transmission of malaria, Chagas disease and HTLV
  • In non-endemic areas, specific risks are posed by the donation of blood by individuals who have lived in or visited areas that are endemic for malaria, Chagas disease or HTLV
  • Specific recipient groups are at risk from the transmission of certain infections such as human cytomegalovirus (CMV).

Reliable epidemiological data are needed to assess the specific risks of transmission by transfusion and of resultant disease. Screening for other TTIs should be considered when there is clear evidence that the safety of the blood supply would be significantly compromised without their inclusion in the screening programme. It should not be implemented until systems are already in place to ensure that all donations are screened for the four major bloodborne infections in a quality-assured manner.

The following issues require consideration before the introduction of screening for TTIs in addition to HIV, HBV, HCV and syphilis:

  • Is the infectious agent readily transmissible through the transfusion of infected blood or blood products?
  • Could the infection result in severe morbidity or mortality in recipients?
  • Is the infection widespread or endemic to the country or region?
  • Can blood donors at risk of the specific infection be identified and deferred through the donor selection process?
  • Is the infectious agent identifiable by blood screening?
  • Is an effective screening assay readily available that can specifically identify infected donations?
  • What are the benefits of screening for an additional TTI in relation to resource and logistics requirements?
  • What might be the impact on the blood supply if such a test is introduced?
  • Are confirmatory assays available to distinguish between true and false positive results?

If indeed considered to be a risk, the specific target marker(s) of the infection then needs to be identified, the appropriate screening strategy and algorithm developed and suitable assays sought.

4.3.1. Malaria

Agent

Malaria is caused by parasites of the Plasmodium species. There are four main types that infect humans: P. falciparum, P. vivax, P. malariae and P. ovale. Malaria is primarily transmitted to humans through the bite of the female anopheles mosquito.

Although always of concern in endemic countries, malaria is increasingly also a matter of concern to blood transfusion services in non-endemic countries. Significant numbers of blood donors from non-endemic countries travel regularly in malarial areas and there is wide migration from endemic areas to non-endemic areas where migrants may then become donors. Malaria is gradually spreading into non-endemic areas or regions where it had previously been eradicated.

Transmissibility

Although primarily transmitted by mosquitoes, malaria is readily transmitted by blood transfusion through donations collected from asymptomatic, parasitaemic donors. The parasite is released into the bloodstream during its lifecycle and will therefore be present in blood donated by infected individuals. The parasites are stable in plasma and whole blood for at least 18 days when stored at +4°C and for extended periods in frozen state.

Screening

There are a number of potential targets for malaria screening and the selection of screening method may depend on whether it is endemic in the country or not. The methods used to identify the presence of malaria employ the following screening targets:

  • Direct detection of parasite by thick film
  • Serological markers:

    Antibody

    Antigen.

Endemic countries

In endemic countries, direct detection of parasite by thick film is often used to identify parasitaemic donations. However, the technique is time-consuming, highly operator-dependent and prone to error. Consequently there is a risk that this approach will not detect lower levels of parasitaemia where transmission may still occur.

High quality, sensitive malaria antigen assays are now readily available and may be better able to identify parasitaemic donations, including those with much lower levels of parasites than are reliably detectable by thick film (39). However, in endemic countries, if screening is even considered, screening strategies are generally complex, combining specific criteria for donor selection and deferral, based on the season, geography and availability of anti-malarial prophylaxis, with laboratory-based screening.

Non-endemic countries

In non-endemic countries, the detection of specific antibody is effective for the screening of donations from individuals identified to be at risk of transmitting malaria. In virtually all cases, the deferral of risk individuals for a period of six months from the date of the last potential exposure combined with malarial antibody testing will prevent the transmission of malaria (39).

RECOMMENDATIONS

Endemic countries

To prevent malaria infection through the route of transfusion in endemic countries:

  1. Donor selection criteria should be developed to identify and collect blood from donors at the lowest risk of infection, both during the malaria season and during the rest of the year.
  2. Donor selection and deferral strategies should be implemented to identify those donors with a current history of malaria or a specific identifiable exposure risk, such as travel to malarious areas. These donors should be deferred for a period defined by the country.
  3. Donor selection and deferral strategies should be developed to identify donors with a history of current malarial infection and defer them for a period of six months after symptoms or treatment have ceased.
    OR
    All donations should be screened for parasitaemia using thick blood films or for evidence of malarial antigen using a highly sensitive enzyme immunoassay.
  4. Transfusion should be followed by the administration of appropriate and effective malarial prophylaxis to all recipients or at least to those recipients at risk of significant disease as a result of transfusion-transmitted malaria.

Non-endemic countries

To prevent malaria infection through the route of transfusion in non-endemic countries:

  1. Donor selection and deferral strategies should be implemented to identify those donors with a current history of malaria or a specific identifiable exposure risk, such as travel to malarious areas. These donors should be deferred for a period defined by the country.
  2. If screening tests are available:
    1. All donors with a history of malaria should be temporarily deferred until six months after symptoms or treatment have ceased and then may be re-instated as donors if there is no evidence of malarial antibody using a highly sensitive enzyme immunoassay.
    2. All donors with an identified malaria exposure risk should be temporarily deferred for a period of six months from their last return from a malarious area and then may be re-instated as donors if there is no evidence of malarial antibody using a highly sensitive enzyme immunoassay.

4.3.2. Chagas disease

Agent

Chagas disease is caused by the parasite Trypanosoma cruzi. Chagas disease is transmitted primarily when the parasite contained in droppings of an infected bug enters the bloodstream through the bite of the primary host, a reduvid bug. However, it can also be transmitted from human to human through the parenteral route by the transfusion of blood or transplantation of tissues from an infected individual.

Chagas disease is geographically restricted and endemic only in Central and South America and parts of Mexico. It is estimated that up to 30% of infected adults die from the chronic effects of Chagas disease in some areas. Up to 20% of infected individuals remain asymptomatic for long periods.

Effective vector control is an important factor in the reduction of the risk of Chagas disease. This has the effect of reducing the disease burden in the population and so reducing the incidence of infections in blood donors. Vector control has proved to be effective in a number of countries in Central and South America, in some resulting in the eradication of all cases of insect-borne infection. Some Latin American countries have eliminated incident cases of primary infection, although a reservoir of infected individuals still remains in the population.

Transmissibility

Although primarily transmitted by an insect vector, Chagas disease is readily transmitted by the transfusion of blood donated by asymptomatic parasitaemic donors. The parasite is released into the bloodstream during its lifecycle and will therefore be present in donated blood from infected individuals. The parasites are stable in plasma and whole blood for at least 30 days when stored at +4°C and for extended periods in frozen state.

Chagas disease is a major concern in endemic countries. It is also of concern to blood transfusion services in some non-endemic countries to which significant numbers of blood donors migrate from regions where Chagas disease is still endemic or where donors travel regularly to endemic areas. Sporadic primary infections have been reported in the southernmost states of the USA. Although not a global problem, many countries have to deal with blood donors who have travelled in Central and South America and therefore need to develop strategies to address this problem.

Screening

Screening for Chagas disease involves the detection of anti-T. cruzi in the donated blood. There are a number of sensitive and reliable assays available and the serology of T. cruzi is well understood (40). In addition, there are antigen detection assays, nucleic acid amplification technology and even xenodiagnosis, although this is clearly not suitable for blood screening.

RECOMMENDATIONS

Endemic countries

To prevent the transmission of Chagas disease through the route of transfusion in endemic countries:

  1. Screening should be performed using a highly sensitive Chagas antibody enzyme immunoassay.

Non-endemic countries

To prevent the transmission of Chagas disease through the route of transfusion in non-endemic countries:

  1. All donors with a history of Chagas disease should be permanently deferred.
  2. If screening tests for Chagas disease are not available, all donors with an identified risk of Chagas disease should be identified and permanently deferred.
  3. If screening tests for Chagas disease are available, all donors with an identified risk of Chagas disease should initially be deferred for six months since their last return from an endemic area. Their subsequent donations should then be screened for evidence of infection using a highly sensitive Chagas antibody enzyme immunoassay.

4.3.3. Human T-cell lymphotropic viruses I/II

Agent

The human T-cell lymphotropic (or leukaemia) viruses I/II (HTLV) are enveloped, single-stranded RNA retroviruses. HTLV is transmitted by the parenteral route and may be found in blood, normally in lymphocytes, and in other body fluids. It is generally not found in plasma or cell-free body fluids.

HTLV is endemic in parts of the world but, in some regions, its incidence and prevalence are low or it may be totally absent. HTLV-I and HTLV-II are two very similar but distinct viruses which are generally considered together because of their similarities. The specific differences include their geographical distribution and clinical disease association. HTLV has a high prevalence in some groups of injecting drug users.

Transmissibility

While HTLV is present in the bloodstream, the levels of the virus itself are variable. In recently infected individuals, virus may be found free in the plasma. Subsequently, free virus is rarely found, the virus being present in the T-lymphocytes. The infectivity of blood and products is reduced but not removed by leucodepletion. As infection is considered to persist for life, screening for anti-HTLV identifies donations that may transmit HTLV but does not in itself indicate the timescale of an infection. However, there is evidence that the pathogenicity of transfusion-transmitted HTLV is low, except in severely immunodeficient recipients (4143).

HTLV is always of concern in endemic countries as well as to blood transfusion services in a number of non-endemic countries. There is significant migration from endemic areas to non-endemic areas where migrants may then become blood donors. In addition, a low level of incident infection may be introduced into a non-endemic country through migration, and the infection may spread horizontally into the non-migrant population or vertically to the children of migrants that were conceived in the non-endemic country.

Screening

HTLV screening involves the detection of specific antibody to both HTLV-I and HTLV-II. Although there is cross-reactivity between HTLV-I and II in a similar way to HIV-1/2, it is incomplete; cross-reactivity to HTLV-I cannot be relied on to detect all cases of HTLV-II. Antibody levels are generally high and, even though the response may vary, antibodies generally persist at a detectable level for life following resolution of the initial acute infection. Combined anti-HTLV-I and -II assays are effective in identifying potentially infectious donations.

RECOMMENDATIONS

To prevent the transmission of HTLV-I/II through the route of transfusion:

  1. In countries in which HTLV is endemic, decisions on introducing screening for HTLV-I and -II should take into consideration the impact on the blood supply.
  2. When implemented, screening for specific anti-HTLV-I/II should be performed using a highly sensitive HTLV-I/II antibody enzyme immunoassay.
  3. Countries in which HTLV is non-endemic should consider screening for evidence of HTLV-I and -II infection prior to the release of blood and blood components for clinical use.

4.3.4. Human cytomegalovirus

Agent

Human cytomegalovirus (CMV) is a herpesvirus, an enveloped DNA virus. CMV is transmitted by the parenteral route and may be found in blood and other body fluids. It is endemic in many parts of the world, although in some regions its incidence and prevalence have declined in recent years as living standards have improved.

Transmissibility

HCMV circulates in the leucocytes and free in plasma during active infection. It subsequently persists latently in leucocytes as well as in other non-circulating body cells and may be released into the bloodstream following reactivation of latent virus. It is thus readily transmitted by blood transfusion, although transmission is generally a concern only when transfusing immunocompromised individuals.

As leucocytes are one of the sites of latency of CMV, pre-storage leucodepletion has been proposed as an additional means of minimizing the risk of CMV transmission. However, while some studies have demonstrated that leucodepletion is just as effective as anti-CMV screening, these have been conducted in populations with a low incidence of CMV infection (4445). In addition, this approach has only been possible where the decision had already been made to introduce leucodepletion for other reasons.

In populations with a higher incidence of CMV, there is a correspondingly higher risk of blood being donated by viraemic individuals. In such cases, leucodepletion will not prevent transmission. Thus, for the majority of countries, anti-CMV screening is still central to the prevention of post-transfusion CMV.

Screening

CMV screening involves the detection of specific antibody to CMV. Antibody levels are generally high and, even though titres may vary, antibodies generally persist at a detectable level for life following resolution of the initial infection.

RECOMMENDATIONS

To prevent the transmission of human CMV infection through the route of transfusion:

  1. CMV-screened whole blood and blood components are not required for immunocompetent individuals.
  2. All whole blood and apheresis donations intended for transfusion to immunosuppressed individuals, neonates and pregnant women should be screened for evidence of CMV infection prior to the release of blood and blood components for clinical use.
  3. Screening should be performed using a highly sensitive CMV total antibody enzyme immunoassay.
  4. Only CMV antibody negative donations should be used for the transfusion of immunosuppressed individuals.
    OR
  5. In the absence of screening, selective leucodepletion may be considered.

4.4. EMERGING AND RE-EMERGING INFECTIONS

Every blood screening programme has to face ongoing challenges. Reports of newly identified infections or re-emerging infections appear regularly in the scientific literature, including reports of their transmission through the route of transfusion. Examples include variant Creutzfeldt Jakob disease, West Nile virus, babesiosis, dengue and chikungunya. There are also infections for which there is a theoretical risk of transmission, but where no cases of transmission have yet been identified or proven, such as severe acute respiratory syndrome (SARS).

While it is likely that new infections will be identified that may be transmissible through transfusion, a cautious and measured response is needed to any apparent new or re-emerging threat to blood safety. Blood transfusion services should develop contingency plans that ensure surveillance for emerging infections, assessment of their transmissibility by transfusion and the actual likelihood of transmission, the diseases associated with transmission, and action to be taken in the event of increasing incidence of infection, including to pandemic level. These plans should also address the potential effects of infection on donors and donor sufficiency, potential recipients, BTS staff and other healthcare staff (46).

Before any consideration is given to introducing screening for a new infection, some important factors need to be taken into account.

  1. Universal screening for HIV, hepatitis B, hepatitis C and syphilis should be in place throughout the country and screening should be applied effectively and consistently in accordance with national standards. Disparities in the standards and quality of screening across a country should be resolved before consideration is given to the introduction of screening for any additional infections.
  2. The actual threat to blood safety should be properly evaluated. The incidence and prevalence of the new infection in the general, blood donor and patient populations should be determined. The disease process associated with the infection needs to be understood, together with its potential impact on the population as a whole and the impact of its transmission by transfusion.
  3. An appropriate screening assay or assays must be available. The technology should be compatible with the current screening strategy and programme, and the resources should be available to implement it. In addition, the ability to confirm reactive results obtained on blood screening should be taken into consideration.

RECOMMENDATIONDS

  1. Laboratory screening for any potential or known transfusion-transmissible infection, other than for the four mandatory infections, should be considered only if:
    • There is a proven risk of transmission of infection to recipients
    • The transmission carries a significant disease risk
    • An appropriate screening assay is available.
  2. Blood screening programmes should include strategies for confirmatory testing and blood donor management.
  3. When there is a proven risk of transfusion-associated transmission but no appropriate screening assays are available, donor selection criteria should be developed to identify and defer potentially infected donors for an appropriate period of time.
  4. When there is a theoretical risk of transfusion-associated transmission and no appropriate screening assays are available, donor selection criteria may be developed to identify and defer potentially infected donors for an appropriate period of time.

4.5. CLINICALLY INSIGNIFICANT TRANSFUSION-TRANSMISSIBLE INFECTIONS

A number of clinically insignificant infections may, on rare occasions, be transmitted by transfusion. These include:

  1. Infections that are not normally transmitted parenterally, but may be transmitted if the blood donor is infected and has a high level of the infectious agent in the bloodstream at the time of donation: e.g. hepatitis A virus (HAV).
  2. Infections that, in theory, can be transmitted, but which are transmitted only very rarely at a significantly lower level than the prevalence or incidence of the infection in the population: e.g. parvovirus B19.
  3. Infections that may be transmitted more frequently, but which then do not give rise to any clinical disease in the recipient: e.g. TT virus.

Routine screening for such infections is generally not practical or cost-effective. The screening tests available, if any, may not be appropriate for blood screening, often being designed primarily to aid the diagnosis of infection in symptomatic individuals. In these situations, the donor selection process is a significant factor in the exclusion of those donors who might harbour these infections in order to prevent them from entering the blood supply.

Copyright © 2009, World Health Organization.

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