Quantification of urinary oxalate by liquid chromatography-tandem mass spectrometry with online weak anion exchange...

Urinary oxalate is commonly measured with an enzymatic assay that is specific but requires a manual clean-up step to reduce ascorbic acid interference. We developed a urinary oxalate assay that uses liquid chromatography-tandem mass spectrometry (LC-MS/MS) with anion exchange chromatography and simple sample preparation.

We added calibrator or urine sample (10 microL) to 10 microL of (13)C2 oxalate and 400 microL of water and performed separation on a Waters OASIS WAX column, flow rate 0.6 mL/min, and then elution for 0.3 min with water containing 2 mmol/L ammonium acetate and 1 mL/L formic acid and for 1.0 min with 750 mL/L methanol containing 20 mL/L ammonia. We detected multiple reaction monitoring transitions m/z 88.6 > 60.5 and m/z 90.5 > 61.5 for oxalic acid and 13C2-oxalate, respectively, with a Quattro micro tandem mass spectrometer in electrospray-negative mode.

Oxalate and 13C2-oxalate eluted at 1.2 min. Mean recovery was 95%, limit of detection 3.0 micromol/L, lower limit of quantification 100.0 micromol/L, linearity to 2212 micromol/L, imprecision <6%, and bias <3% at 166, 880, and 1720 micromol/L. Oxalate eluted after the main area of ion suppression. Mean response ratios for urine and aqueous samples, enriched at 200 and 1000 micromol/L, were 3.7% and 5.4%, respectively. No interference was observed from other organic acids. Passing and Bablock regression analysis comparing the Trinity Biotech enzymatic reagent set and LC-MS/MS showed LC-MS/MS = 1.06 (enzymatic assay) -21.2, r = 0.964, n = 110. Bland Altman analysis showed general agreement, with a mean bias of -1.9 mumol/L.

This LC-MS/MS assay is applicable for quantifying urinary oxalate excretion.