Number of colony forming units in urine at 35–37 weeks’ gestation as predictor of the vaginal load of Group B Streptococci at birth

Objective: To evaluate GBS colony numbers in the urine at 35–37 weeks’ gestation to predict the load of GBS-colonization of the vagina at birth. Study design: In this prospective observational study, we included 902 unselected pregnant women. Exposure was GBS colony forming units (CFU) per mL urine at 35–37 weeks’ gestation. Outcome was vaginal GBS colonization at birth as assessed by a semi-quantitative culture of a vaginal swab sample (negative, +1, +2, +3). Results: Bacteriuria with GBS at 35–37 weeks’ gestation performed with a sensitivity of 30% concerning any degree of vaginal GBS colonization at birth (31 of 104 cases); 19% for light (+1), 17% for medium (+2), and 52% for high load (+3) vaginal GBS colonization. The colony count in case of GBS bacteriuria at 35–37 weeks’ gestation performed with positive predictive values of 35% for <10 ⁴ CFU/mL, 70% for 10 ⁴ CFU/mL, and 67% for >10 ⁴ CFU/mL. Conclusion: Even though the urinary GBS CFU at 35–37 weeks’ gestation is strongly associated with a high load of vaginal GBS colonization intrapartum, it may not perform satisfactorily as a standalone-screening marker for risk of early-onset GBS disease.