The accuracy of late antenatal screening cultures in predicting genital group B streptococcal colonization at delivery

Abstract

Objective: To determine the accuracy of late antenatal anogenital cultures in predicting group B Streptococcal colonization at delivery. Methods: Swabs of the vagina and rectum were obtained from 826 women during routine prenatal visits at approximately 35–36 weeks' estimated gestation. The same women were recultured at admission for delivery. Swabs were cultured in broth media. Test performance indices were calculated using culture status at the time of delivery as the reference. Based on the sensitivity and specificity of antenatal cultures derived from analysis of this study population, we estimated predictive values of late antenatal cultures for a range of group B streptococcal carriage rates. Results: Group B streptococci were identified in specimens from 219 of 826 women (26.5%). The sensitivity of late antenatal cultures for identifying colonization status at delivery was 87% (95% confidence interval [95% CI] 83–92). Specificity was 96% (95% CI 95–98). Positive predictive value was 87% (95% CI 83–92), and negative predictive value was 96% (95% CI 95–98). Test performance was similar from 1–5 weeks before delivery, but declined when 6 or more weeks had elapsed between the antenatal culture and delivery. Among patients cultured 6 or more weeks before delivery, sensitivity was only 43%, specificity 85%, and positive and negative predictive values were 50 and 81%, respectively. We estimated positive and negative predictive values of 85 and 97% for a colonization rate of 20%, and 79 and 98% for a colonization rate of 15%. Conclusion: Anogenital cultures in broth media obtained during the late antenatal period are accurate in predicting group B streptococcal colonization status at delivery in term parturients, and they perform significantly (P < .01) better than cultures collected 6 or more weeks before delivery.