Mass spectrometric-selected ion monitoring assay for an oxalate absorption test applying [13C2]oxalate.

In order to assess the significance of the intestinal absorption of oxalate from food for the hyperoxaluria of the individual patient, an oxalate absorption test has been developed using doubly 13C-labelled oxalate and a gas chromatographic selected ion monitoring mass spectrometric assay. This test has been applied to volunteers and patients with urinary stones. The percentage of dose absorbed (range 1-48%) could be determined with a coefficient of variation of 15.2%. The assay to measure doubly 13C-labelled oxalate in the presence of unlabelled oxalate in urine, using the homologue malonic acid as internal standard, is described.