A single-point estimate of the milk to plasma (M/P) drug ratio presupposes an invariant ratio. This may lead to erroneous conclusions about the amount of drug excreted in breast milk. A variance of the M/P ratio with regard to time has been shown for acetaminophen under standardized biologic conditions in the lactating goat. This ratio is potentially confounded by such factors as route of administration, dose amount and duration of exposure to the drug. Each factor was evaluated by computer simulations of the M/P ratio and by derivation of a ratio from empirical data in the goat. Route of administration was found to influence both the pattern and magnitude of the M/P ratio after acetaminophen dosing in the goat. Similar, but less, effects were found for duration of dosing. Dose itself did not influence the ratio. A temporal basis exists for each confounding factor. We thus propose that the M/P ratio is fundamentally variant with time under a standard set of biologic conditions. The contributing variance of confounding factors must be assessed if drug in milk dosing of the infant is to be accurately evaluated by use of the M/P ratio.

Keywords:
Breast milk, Milk/plasma ratio, Pharmacokinetics, Acetaminophen, Goat
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1987
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