Enzymic assay for oxalate in unprocessed urine, as adapted for a centrifugal analyzer

In this automated modification of the oxalate decarboxylase method, oxalate can be measured (12 per hour) in acidified but otherwise unprocessed urine. Standard curves are linear up to at least 2.5 mmol/L. When 0.50 mmol of oxalate was added per liter to samples of 18 patients' urines, a mean analytical recovery of 98.5% (SD 3.6%) was obtained. Within-series CVs were 3.4 and 1.0%, between-series CVs 7.3 and 2.7% (n = 15) for oxalate concentrations of 0.31 and 0.61 mmol/L. The lower limit of detection is 25 mumol/L. Concentrations measured with this "direct" method correlated well (r = 0.95) with those measured after precipitation with calcium and ethanol and resolubilization in dilute sulfuric acid. For 17 healthy volunteers the mean urinary excretion of oxalate was 0.37 (SD 0.14) mmol/24 h.