For this direct colorimetry of urinary oxalate, commercially available oxalate oxidase (EC 1.2.3.4) is used. The urine is first diluted, to diminish the effect of interfering substances. Analytical recovery of oxalate from urines with five different oxalate concentrations (0.4 to 2.0 mmol/L) ranged from 92 to 109% (mean 99%). The within-day and between-day precision (CV) of the method for a wide range of oxalate concentrations averaged better than 10%. There is good correlation (r = 0.977) between this enzymatic method (y) and the chemical method of Hodgkinson and Williams (x) [Clin Chim Acta 36: 127-132, 1972], the regression equation being y = 1.014x + 0.061. Urines with added ascorbate give falsely increased results. The proposed method is inexpensive and simple to perform.
The management of patients who have taken large amounts of acetaminophen is often determined by the plasma level of the drug in relation to the time after ingestion. Nomograms or tables defining the risk of the hepatic damage for a given level at a particular time usually refer to the plasma-free acetaminophen concentration. It is important therefore that the assay used measures only free acetaminophen and not its inactive conjugates of glucuronide and sulfate. Two simple colorimetric methods were compared to assess the interference of acetaminophen conjugates. The method of Frings and Saloom measured both conjugates. Furthermore, these conjugates are not completely hydrolyzed, averaging about 22 and 85% hydrolysis for the glucuronide and sulfate conjugates, respectively. The method of Glynn and Kendal is little affected by acetaminophen conjugates and gives a better measure of the free drug. Interference by salicylates, although slight, can be corrected using modifications in the method.
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