Quantitative Determination of Fosfomycin in 10 μL of Plasma and Dialysate by Hydrophilic Interaction Liquid Chromatography Electrospray Ionization Mass Spectrometry

Fosfomycin is an antibiotic with a broad spectrum of activity against many multidrug-resistant bacterial strains. It is mainly excreted unchanged by the kidneys, and its half-life therefore depends on kidney function which varies considerably among individuals, and within individuals over time. Proper fosfomycin dosing thus depends on assaying blood concentration of the drug. We developed and validated a simple, sensitive and specific chromatography assay, which was coupled to electrospray ionization mass spectrometry for determination of fosfomycin. Separation of fosfomycin was based on the method of the hydrophilic interaction liquid chromatography; specifically, plasma and dialysate samples were acidified and the protein precipitated with acetonitrile. The calibration curves showed excellent coefficients of determination (R-2 > 0.999) over the relevant concentration range of 25-700 mu g/mL. Intraday precision was 1.1-1.2% and accuracy was -5.9% to 0.9% for quality control samples. Interday precision was 2.9-3.4% and accuracy was -3.7% to 5.5%. Extraction recovery was >= 87% and matrix effects ranged from 2.2% to 4.3%. After laboratory validation, the method was successfully applied to clinical samples.

Automated summary

Fosfomycin is an antibiotic effective against multidrug-resistant bacterial strains, with dosing depending on individual kidney function. A chromatography assay coupled with mass spectrometry was developed to determine fosfomycin concentration, showing high precision and accuracy for quality control samples. The method was successfully applied to clinical samples after laboratory validation.