The Accuracy of Late Third-Trimester Antenatal Screening for Group B Streptococcus in Predicting Colonization at Delivery

We reevaluated the accuracy of antenatal group B streptococcal (GBS) culture results in predicting colonization at the time of delivery in a general practice setting. Patients who had late third-trimester antenatal GBS cultures were prospectively identified. A repeat GBS culture was performed when admitted in labor using a strict protocol and laboratory analysis. Sensitivity, specificity, and positive and negative predicative values were calculated. In comparing the office culture results to the intrapartum cultures in 1472 patients, the sensitivity was 51%, specificity 94%, positive predictive value 67%, and negative predictive value 88%. The antenatal positive culture rate of 15.4% was significantly lower than the 20.1% intrapartum positive culture rate. There were 144 patients (9.8%) who had false-negative antenatal culture results. Through office survey, several aspects of the recommended antenatal procedures were not followed. The results support the previously reported high specificity and negative predictive values for this test. The near 10% false-negative rate with the significant difference between the antenatal versus intrapartum positive culture rate highly suggests that late third-trimester culture accuracy may be affected if the specified procedures are not completely followed, including the culturing technique, the use of recommended transport medium, and the laboratory culture protocol.