Fluorimetric and microbiological assays of erythromycin concentrations in plasma and vaginal washings

Using a microbiological and a fluorimetric assay to determine penetration of erythromycin into vaginal fluids, concentrations were measured in plasma from nine men one hour after a single oral dose of erythromycin stearate 2 g and in vaginal washings and plasma samples taken simultaneously from 11 women two hours after the last dose of a 10-day course of erythromycin stearate (250 mg four times daily). Both assay methods gave accurate and reproducible results in plasma but only the fluorimetric method was capable of measuring concentrations of erythromycin in vaginal washings. The latter method had many advantages in estimating drug concentrations in body fluids such as vaginal washings and the results from it may provide an index of tissue penetration and of patient compliance in adhering to drug regimens.