Are postnatal ampicillin levels actually related to the duration of intrapartum antibiotic prophylaxis prior to delivery? A pharmacokinetic study in 120 neonates

Objective To assess ampicillin levels according to the duration of intrapartum antibiotic prophylaxis (IAP). Design Prospective cohort single-centre study. Setting Tertiary care centre (Modena, Italy). Patients 120 neonates=35 weeks' gestation exposed to IAP. Interventions Neonates were divided into four groups, according to the duration of IAP prior to delivery: group 1 (n=30; <1 hour), group 2 (n=30; =1 and <2 hours), group 3 (n=30; =2 and <4 hours) and group 4 (n=30; =2 doses, =4 hours). Main outcome measures Blood samples were collected at delivery (from the umbilical cord) and at age 4 hours (from a peripheral vessel). Results Median duration of IAP was 121 min (range 7-2045 min). Median ampicillin levels in umbilical cord blood were 10.4 μg/mL (IQR 6.4-14.9) and in peripheral blood were 4.7 μg/mL (IQR 2.8-6.4μg/ mL). Umbilical cord blood levels reached a peak approximately 30 min after IAP and then declined significantly (p<0.001). Peripheral blood levels did not differ among study groups. Neonates exposed to a full loading dose (n=115) had peripheral blood levels 2.5-70 times higher than the minimal inhibitory concentration for group B streptococcus. There was no relationship between neonatal ampicillin concentrations and the duration of IAP prior to delivery (ß=-0.0003, 95% CI -0.02 to 0.001, p=0.680). Conclusions Ampicillin levels reach a peak in the umbilical cord blood within 30 min of intrapartum administration. After a full loading dose, bactericidal levels persist for at least 4 hours after birth and seem independent of the duration of IAP prior to delivery.