A validated method for the quantification of fosfomycin in human plasma by liquid chromatography-tandem mass spectrometry.

Fosfomycin is a small, hydrophilic antibiotic drug with activity against Gram-positive as well as Gram-negative pathogens. It is in increasing use in intensive care units as a last line antibiotic since it shares no cross-resistance with other antibiotics. It is not metabolized and plasma levels are dependent on renal excretion rate and renal replacement therapy such as hemofiltration or hemodialysis. Measurement of fosfomycin plasma concentrations is therefore highly desirable in order to optimize dosing. We have developed a method for the quantification of fosfomycin in human plasma using HILIC chromatography on a silica stationary phase and tandem mass spectrometric detection. Sample preparation consisted only of protein precipitation without derivatization. Propylphosphonic acid was used as internal standard. Two calibration ranges from 15 to 150μg/ml and 100 to 750μg/ml were necessary to cover the whole range of plasma concentrations expected from intensive care patients. Intraday precision ranged from 4.0% to 6.4%, depending on the concentration level, with accuracies ranging from -1.1% to 11.5%. The corresponding interday precisions and accuracies were 2.0-11.0% and 0.6-7.8%, respectively. Fosfomycin was stable in human plasma under all storing conditions relevant for clinical samples. First experiences with this method in clinical routine use confirmed the applicability and ruggedness of the analytical procedure.