Intrapartum Antibiotic Prophylaxis 2: Positive Predictive Value of Antenatal Group B Streptococci Cultures and Antibiotic Susceptibility of Clinical Isolates

Objective: To estimate the probability of positive intrapartum group B streptococcus cultures among women previously identified as carriers of this organism, and to estimate the susceptibility of group B streptococci to six commonly used antibiotics.

Methods: We performed a prospective cohort study of women identified as carriers of group B streptococci by current pregnancy genital tract (group 1) or urine cultures (group 2), or a positive culture in a prior pregnancy (group 3). Intrapartum culture specimens were obtained, and isolates were tested for susceptibility to six antibiotics using the agar disk diffusion technique.

Results: Intrapartum cultures were positive for 68% (62, 73), 61% (49, 72), and 48% (36, 60) of groups 1 (n = 249), 2 (n = 69), and 3 (n = 59), respectively. Cultures were positive in 67% (61, 73) of women in group 1 whose cultures were done 42 days or less before delivery (n = 218). The proportion of isolates (n = 239) susceptible to penicillin, ampicillin, cefazolin, and vancomycin was 100% (98, 100). The proportion susceptible to clindamycin and erythromycin was 91% (87, 94) and 79% (73, 84), respectively.

Conclusion: The positive predictive value of antenatal group B streptococci cultures is lower than was previously reported. Clindamycin and erythromycin are not optimal agents for prophylaxis against early-onset neonatal group B streptococcal infection in patients who are allergic to penicillin.