Validation of a LC-MS/MS assay for citric acid, cysteine and oxalic acid determination and its application to explore pre-analytical sample storage

Citrate, oxalate and cystine in 24-h urine are considered to be associated with the incidence and recurrence risk of urinary stone disease (USD). An evaluation of the LC-MS/MS kit for simultaneous quantification of the three analytes was undertaken. & Methods: The analytical performance of the kit was investigated based on FDA, EMA and CLSI guidelines. To promote the standardization of sample storage, this kit has been applied to perform systematic pre-analytical stability study of these analytes in urine. This method was validated with good linearity with accuracy of 93.1%–104 %. Intra-day and inter-day imprecision were ≤5.55 % and 5.34 %, respectively. Recoveries of citrate, oxalate and cystine added to clinical samples were in the range of 92.0–103 %, 94.8–100 % and 99.0–107 % with CV ≤ 5.52 %. It was recommended that urine preserved with hydrochloric acid could be preferable in consideration of both reliable test results and neglected sample heterogeneity. This kit is suitable for measurement of citrate, oxalate and cystine for understanding the etiology of urinary stones, and the proper storage of urine samples is crucial for the correctness of the test results.