A GBS culture collected shortly after GBS prophylaxis may be inaccurate

To determine whether a vaginal-rectal culture obtained after antibiotic therapy has begun accurately detects pre-existing colonization with group B streptococcus (GBS).

A prospective cohort study of women presenting at in labor who were known to be colonized with GBS were recruited. A GBS culture was obtained prior to administration of intravenous penicillin prophylaxis and repeated 2 hours following the first dose of penicillin. The two results were compared.

Eighty subjects were recruited. Complete results were obtained for 61 (76 %) subjects. Of these, 47 (77 %) had a GBS positive on initial culture. Persistence of GBS 2 hours after antibiotic exposure was seen in 30/47 (64 %). Conversion from GBS positive to GBS negative status was seen in 17/47 (36%).

A vaginal-rectal culture for GBS performed after antibiotic prophylaxis has commenced may not accurately reflect a patient's GBS colonization status.