A validated method for the quantification of fosfomycin on dried plasma spots by HPLC-MS/MS: application to a pilot...

Quantification of fosfomycin in the plasma samples of patients is the basis of clinical pharmacokinetic studies from which evidence based dosing regimens can be devised to maximise antibiotic effectiveness against a pathogen. We have developed and validated a LC-MS/MS method to quantify fosfomycin using dried plasma spot sampling. Following HILIC chromatography, fosfomycin and ethylphosphonic acid, used as internal standard, were measured using negative-ion multiple reaction monitoring. The method was linear over the calibration range of 5-2000mg/L of fosfomycin. Intra-day assay results for dried plasma spot quality control samples at 15.6, 79.9 and 1581mg/L of fosfomycin had precision of ±4.2, 8.2, and 2.0%, respectively, and accuracy of +3.9, -0.1, and -1.2%, respectively. Recovery of fosfomycin from dried plasma spots was calculated as 83.6% and the dried plasma spot samples were found to be stable stored at room temperature for three months and when stored for four hours at 50°C. A Bland-Altman plot comparing DPS to plasma sampling found a negative bias of 16.6%, with all but one sample within the mean limits of agreement (-2.6 to 30.6%). Dried plasma spot sampling provides a useful tool for pharmacokinetic research of fosfomycin.