Intrapartum tests for group B streptococcus: accuracy and acceptability of screening

Abstract

To assess the accuracy and acceptability of polymerase chain reaction (PCR) and optical immunoassay (OIA) tests for the detection of maternal group B streptococcus (GBS) colonisation during labour, comparing their performance with the current UK policy of risk factor-based screening. Diagnostic test accuracy study. Fourteen hundred women in labour at two large UK maternity units provided vaginal and rectal swabs for testing. The PCR and OIA index tests were compared with the reference standard of selective enriched culture, assessed blind to index tests. Factors influencing neonatal GBS colonisation were assessed using multiple logistic regression, adjusting for antibiotic use. The acceptability of testing to participants was evaluated through a structured questionnaire administered after delivery. The sensitivity and specificity of PCR, OIA and risk factor-based screening. Maternal GBS colonisation was 21% (19-24%) by combined vaginal and rectal swab enriched culture. PCR test of either vaginal or rectal swabs was more sensitive (84% [79-88%] versus 72% [65-77%]) and specific (87% [85-89%] versus 57% [53-60%]) than OIA (P < 0.001), and far more sensitive (84 versus 30% [25-35%]) and specific (87 versus 80% [77-82%]) than risk factor-based screening (P < 0.001). Maternal antibiotics (odds ratio, 0.22 [0.07-0.62]; P = 0.004) and a positive PCR test (odds ratio, 29.4 [15.8-54.8]; P < 0.001) were strongly related to neonatal GBS colonisation, whereas risk factors were not (odds ratio, 1.44 [0.80-2.62]; P = 0.2). Intrapartum PCR screening is a more accurate predictor of maternal and neonatal GBS colonisation than is OIA or risk factor-based screening, and is acceptable to women.