Background: This study compared three methods of estimating the daily dose of fluoxetine to nursing infants and the relationship between these estimates and infant serum concentrations.
Methods: Breast milk and infant serum concentrations of fluoxetine and norfluoxetine were obtained from 10 nursing mother-infant pairs. Quantification of daily infant dose was determined by three methods: 1) collection of the total volume of breast milk over 24 hours and determination of the average breast milk concentration (Baby's Total Daily Dose); 2) determination of the maximum and minimum breast milk concentrations during 24 hours and an estimated milk consumption of 150 mL/kg/day (Atkinson Model); and 3) determination of the gradient of excretion of medication into breast milk at a specified time after the maternal dose, applying this gradient to each nursing collection and summing the values for 24 hours (Mathematical Model). The relationship between the 24-hour medication dose, obtained from each method, and the infant serum concentrations was examined.
Results: A total of 177 breast milk and 10 infant serum samples were collected. An estimate of the infant daily medication dose obtained by the Mathematical Model was the best predictor of total infant serum concentration.
Conclusions: Breast milk analysis may allow one to determine whether medication concentrations will be detectable in an infant, eliminating the need for an infant serum concentration. Although the Mathematical Model seems to reflect infant serum concentration most accurately, all three methods suggest that the maximum dose that a nursing child receives over the course of a year equals as much as 120 mg, or 160 +/- 47% of the maternal daily dose.