Introduction Recent data have raised questions over the efficacy of azithromycin 1g for the treatment of chlamydia infection. In order to measure effective absorption, we developed a protocol to quantify the concentration of azithromycin using liquid chromatography and tandem mass spectrometry (LC-MS/MS) in self-collected high-vaginal swabs.

Methods Ten healthy women were asked to self-collect a high-vaginal swab (baseline) prior to taking a 1g dose of azithromycin. A blood sample was collected four hours later to determine plasma concentrations of azithromycin. Participants then self-collected a high vaginal swab each day for a further 9 days. All swabs were preserved in 1ml of 100% Methanol and stored at –80°C prior to analysis. One ml of chloroform containing 10mg/ml of Leucine enkephalin as an internal standard was added to extract azithromycin. Azithromycin concentrations were calculated using a validated LC-MS/MS method. Data were normalised to the internal standard and to membrane lipid concentrations, measured in the same samples using LC-MS/MS.

Results Azithromycin was detected at varying concentrations in all 10 women in all post-treatment samples. The highest average normalised azithromycin concentration of 953ng/ml (range = 267–2200ng/ml, standard error of mean (sem) = 181ng/ml) was detected on day 2 post-treatment. The lowest average azithromycin concentration was 164ng/ml (range = 51–387ng/ml, sem = 42ng/ml), 9 days post-treatment. The average concentration of azithromycin detected in blood samples was 339ng/ml (range = 107–628ng/ml, sem = 57ng/ml). In 9/10 women azithromycin concentrations remained above 64ng/ml, the hypothesised mean inhibitory concentration (MIC) of azithromycin for chlamydia, for the entire 9 days.

Conclusion We have validated a method for detecting the azithromycin concentration in self-collected high-vaginal samples using LC-MS/MS. Azithromycin concentrations remained above the reported MIC of 64ng/ml for up to 9 days post-treatment in high-vaginal swabs from 10 healthy women.