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

To assess ampicillin levels according to the duration of intrapartum antibiotic prophylaxis (IAP).

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 (β=â '0.0003, 95% CI â '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.