Development and validation of an HPLC–MS method for a simultaneous measurement of oxalate and citrate in urine

Abstract Objectives An increased urinary oxalate and reduced urinary citrate are considered major risk factors in the formation of calcium oxalate kidney stones. In this work, an HPLC–MS method is presented for the simultaneous measurement of oxalate and citrate in urine. Methods Sample preparation was carried out using a liquid–liquid extraction with ethyl acetate. Chromatographic separation was performed in a C18 column by gradient elution with methanol and 1 M formate buffer at 35 °C. Citrate and oxalate were monitored on a single-quadrupole MS system. Results The method was linear in the concentration range of 0.5 mg/L to 450 mg/L for oxalate and from 2.5 mg/L to 950 mg/L for citrate. The Lower Limit of Measurement was 0.56 mg/L for oxalate and 2.5 mg/L for citrate. The within-day imprecision was 6% for oxalate and 3% for citrate, and the between day imprecision was lower than 15% for both analytes. LC–MS method was compared with capillary electrophoresis and it was shown that both methods were interchangeable to measure oxalate, but not citrate. Conclusions HPLC–MS method is a good approach to measure oxalate and citrate in 24-hour urine, and it is applicable in clinical routine for patients with recurrent stone formation.