Wilson, David M.

Abstract Malignant melanoma, a disease that is increasing in occurrence and medical concern, is characterized by the excretion of melanogens. Two qualitative tests are recommended for melanogen detection, the Thormählen test and the ferric chloride test. We evaluated the laboratory and clinical performance of these tests by subsequently re-evaluating 201 urine samples that had been submitted for routine melanogen analysis. We used (a) Thormählen, (b) ferric chloride, (c) small-scale thin-layer chromatography, and (d) spectrophotometry. Nearly 30% of Thormählen test results were equivocal. The ferric chloride test was of no value in itself or in categorizing equivocal Thormählen results as positive or negative. The small-scale chromatography was irreproducible. Prompt scanning of the Thormählen reaction product was helpful in classifying equivocal results. History review of 121 histopathologically diagnosed melanoma patients indicated that these qualitative assays were of no clinical value in the diagnosis or monitoring of melanoma patients.

Abstract The measurement of oxalate in urine and plasma continues to be difficult, particularly in the presence of ascorbate. We have modified and validated a colorimetric assay involving the use of oxalate oxidase (EC 1.2.3.4). Modification of an HPLC spectrophotometric detector improved sensitivity (to as much as 1000-fold that of conventional spectrophotometers) and allowed measurement of oxalate concentrations less than 1 mumol/L. This provided more than enough sensitivity for measurement of normal concentrations of plasma oxalate. We established reference values for oxalate concentrations in urine and plasma, studied sample handling, and established conditions to avoid ascorbate interference in urine and plasma measurements. Mean analytical recovery of [14C]oxalate from plasma to the filtrate was 86 (SD 10)%; recovery of unlabeled oxalate from filtrate was 87 (SD 9)%. Urinary oxalate excretion rates in apparently healthy controls were 0.11-0.46 mmol/24 h. Plasma concentrations in control subjects were 2.5 (SD 0.7) mumol/L, similar to concentrations determined by recent gas chromatographic and isotope dilution methods. Frozen and acidified urine samples showed no interference from ascorbate when excess ascorbate was avoided. Ingestion of 2 g of ascorbate daily did not increase urinary oxalate in healthy control subjects, but during storage ascorbate was converted to oxalate in all conditions tested.