10.9.3What are the diagnostic tests available for antenatal detection of GBS carriage and how do they compare in terms of specificity, sensitivity, and cost-effectiveness?

StudyRef.PopulationInterventionOutcomesResultsCommentsStudy typeEL
Schrag et al., 2002443A stratified random sample of 5144 live births were selected from 629,912 live births from 1998 and 1999 from 8 geographical areas in the USA. All births of infants with early-onset infection were included in the sample (n = 312)Universal culture screening vs. screening by assessment of clinical risk factors to identify candidates for intrapartum antibiotics for GBSPrevention of early onset GBS disease in infants less than 7 days oldRisk of early-onset disease lower in universally screened group: adjusted relative risk 0.46 (95% CI 0.36 to 0.60)
After excluding all women with risk factors and adequate time for prophylaxis, adjusted relative risk was still similar: 0.48 (95% CI 0.37 to 0.63)
Spieker et al., 1999442240 pregnant women at 28 weeks of gestation in Florida, USAPatients received written instructions on how to obtain rectovaginal swab and obtained own swab. Physician also obtained swab
Reference standard was any culture obtained by physician or women found to be positive
Cultures positive for GBS24% (24/240) cultures positive for GBS
patient sensitivity 79%, physician sensitivity 83%, p = 0.365
Molnar et al., 1997441163 women presenting for their 26 to 28 week antenatal care visit at five family physician offices and eight obstetricians at a hospital in Toronto, Canada from November 1995 through March 1996Patient survey about who women would prefer to do their swabs; vaginal/anorectal swab collected by patient on self and vaginal/anorectal swab collected by physician on same woman
Any culture positive for GBS obtained by women or physician used as reference standard
Comparison of GBS detection rateOverall prevalence of maternal GBS carriage: 24% (39/163) (95% CI 17% to 30%)
Concordance between physician- and patient-collected swabs was 95% (95% CI 92% to 98%)
Patients identified 38 cases for sensitivity of 97% (lower 95% CI 92%); physicians identified 32 cases for sensitivity of 82% (95% CI 70% to 94%)
From 161 surveys, 54 (34%) of women preferred to do their own swab, 66 (41%) were indifferent and 41 (26%) preferred physician to do their swab
Boyer et al., 19834405586 cultures from pregnant women at obstetric practices in Chicago, USA from April 1979 to Sept 1981Cultures from vagina and rectum
Colonies with suggestive haemolysis or morphology identified as GBS with CAMP test
Women with positive prenatal cultures, cultures obtained again intrapartum and within three days of delivery
200 women with negative prenatal cultures also recultured
Value of prenatal culture for identifying GBS colonisation status at deliveryOverall, 22.8% (1272/5586) women were carriers of GBS
In colonised women, rectal cultures were more frequently positive than vaginal cultures (82% vs. 65%)
575/1272 GBS carriers were restudied at delivery.
Of 182 antenatal positive vaginal and rectal cultures, 132/182 (73%) were positive at delivery
Of 67 antenatal positive vaginal cultures, 46/67 (69%) were positive at delivery
Of 144 antenatal positive vaginal cultures, 86/144 (60%) were positive at delivery
Of 200 antenatal negative vaginal and rectal cultures, 17/200 (9%) were positive at delivery
Estimated sensitivity and specificity of prenatal culture: 70% and 90%, respectively
182/575 recultured women with incomplete or unquantified cultures were excludedEV3
Yancey et al., 1996439826 women attending antenatal clinics in the USAVaginal and rectal swabs at approx 35 to 36 weeks gestation and again at deliveryOverall colonisation rate
Test performance by culture-delivery interval
GBS identified in 219/826 (26.5%) of women
In cultures obtained 1 to 5 weeks before delivery, sensitivity 87% (95% CI 83% to 92%), specificity 96% (95% CI 95% to 98%)
Among patients cultured 6 weeks or more before delivery, sensitivity 43% and specificity 85%

From: Evidence tables

Cover of Antenatal Care
Antenatal Care: Routine Care for the Healthy Pregnant Woman.
NICE Clinical Guidelines, No. 62.
National Collaborating Centre for Women's and Children's Health (UK).
London: RCOG Press; 2008 Mar.
Copyright © 2008, National Collaborating Centre for Women’s and Children’s Health.

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