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.