Optimal timing of ampicillin administration to pregnant women for establishing bactericidal levels in the prophylaxis of Group B Streptococcus

Objective: The purpose of this study was to determine the time necessary to achieve and maintain bactericidal concentrations of ampicillin in the cord blood. Study design: This was a prospective study in which women scheduled for an elective cesarean section were given intravenous ampicillin before the procedure. Cord and simultaneous maternal blood samples were collected at the time of delivery. Serum ampicillin levels were analyzed by high-performance liquid chromatography. Results: The ratio of cord blood to maternal serum ampicillin concentration was found to increase over time in a linear fashion. There was no correlation between maternal body mass index (22.3 to 48.3 kg/m(2)) and ampicillin cord blood concentrations. All cord serum samples far exceeded minimal bactericidal concentrations reported for Group B Streptococcus (0.25 to 2.0 mu g/mL), even at 338 minutes following administration. Conclusion: Bactericidal levels of ampicillin in the cord blood are rapidly achieved within 30 minutes of administration of ampicillin to the mother. The increase in the ratio of cord to maternal serum ampicillin levels is directly related to time, suggesting a decrease in the clearance of ampicillin in the newborns as compared to the mothers. The cord blood ampicillin concentration exceeds the maternal concentration and both continue to be above the minimal bactericidal concentrations at 5.6 hours after administration. No relationship was observed between the maternal body mass index and ratio of cord to maternal serum concentrations of ampicillin. (c) 2006 Mosby, Inc. All rights reserved.