Liquid chromatography–tandem mass spectrometry method for routine measurement of oxalic acid in human plasma

A solid phase extraction (SPE)-LC–MSMS method for the routine determination of oxalic acid (OX) in plasma, a diagnostic marker of primary hyperoxaluria (PH), was developed and validated. The normal range of OX was found to be 3–11 μmol/L (n = 67), with no differences attributable to gender or age. The effect of pre-analytical factors on the in vitro production of OX was investigated, and plasma was found to be stable for 1–2 h at room temperature, less after ingestion of vitamin C; the process was not completely stopped by preservation at either −20 or −70 °C.