Gentamicin excretion and uptake from breast milk by nursing infants

To investigate the excretion of gentamicin into human breast milk and resulting serum gentamicin levels in nursing newborn infants.

Women delivered by cesarean received gentamicin, 240 mg/day (80 mg intramuscularly three times a day) for 5 days postpartum. On day 4, maternal serum samples were collected 1 and 7 hours after gentamicin administration. Milk samples were collected 1, 3, 5, and 7 hours following administration. The infants were fed 1 hour after gentamicin administration, and serum samples were collected from the newborns 1 hour later. The concentrations of gentamicin were measured by a fluorescence polarization immunoassay.

The mean (+/- standard deviation) maternal serum gentamicin levels at 1 and 7 hours were 3.94 +/- 1.12 and 1.02 +/- 0.78 microgram/mL, respectively. Milk gentamicin levels were: 0.42 +/- 0.26, 0.48 +/- 0.17, 0.49 +/- 0.17, and 0.41 +/- 0.25 microgram/mL at 1, 3, 5, and 7 hours, respectively. The mean milk:plasma gentamicin ratios were 0.11 and 0.44 at 1 and 7 hours, respectively. The correlation between maternal peak serum levels and milk levels was not statistically significant (P > .05). Detectable (above 0.27 microgram/mL) gentamicin levels were found in five of the ten newborn serum samples, with a mean level of 0.41 +/- 0.05 microgram/mL.

Gentamicin is transferred into breast milk, and half of nursing newborn infants have detectable serum gentamicin levels.