A continuous-flow assay for measuring oxalate in urine is described. Covalently attached oxalate oxidase (EC 1.2.3.4) is used to oxidise the oxalate anion to carbon dioxide and hydrogen peroxide. The formed hydrogen peroxide is measured colorimetrically (A58n) with an established reaction using horseradish peroxidase (EC 1.11.17), 3-methyl-2-benzothiazolinone hydrazone (MBTH) and 3-dimethylaminobenzoic acid (DMAB). Ascorbate interference is eliminated by treating the urine sample with sodium nitrite prior to assaying. The assay is accurate (mean recovery of added oxalate in spiked urine sample is 93 ± 11%), sensitive (detection limit 1·0 μmol/L), reproducible (within-batch CV 3·5%; between-batch CV 5%) and relatively rapid (15 samples/h). This assay correlates well (R = 0·99) with another established enzymatic method (using oxalate decarboxylase).
Calcium oxalate and calcium phosphate crystalluria have been measured chemically in 1,173 urine samples whose chemical compositions were also analysed. The importance of urinary oxalate as a determinant for calcium oxalate crystalluria was confirmed. Significant concentrations of calcium oxalate crystals may be present in urine even though the crystals are too small for detection by light microscopy or by many particle-counting methods. Calcium phosphate crystals in urine always contain a small proportion of calcium oxalate. Results in various clinical situations are reviewed.
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