The phosphate concentrations were measured in 41 patients who had multiple myeloma with paraproteinaemia using four different methods to compare the incidence of pseudohyperphosphataemia. The direct acid/molybdate method produced the highest number of anomalous results. The erroneously high phosphate concentration was attributable to the presence of turbidity in the reaction mixture. No association was found between paraprotein type and occurrence of turbidity. The This is an enzymatic method in which the phosphate ions react with inosine in the presence of nucleoside phosphorylase to produce ribose-l-phosphate and hypoxanthine. The latter is oxidised to xanthine then to uric acid, and the hydrogen peroxide formed measured as the quinoneimine product when it reacts with 4-aminophenazone. Absorbence was measured at 550 nm. The reagents were obtained as a pack from Miles Ltd, Stoke Court, Stoke Poges, Buckinghamshire. The data obtained were not normally distributed but logarithmic transformation resulted in acceptable normalisation of the data. The paired t test was used to analyse log transformed phosphate concentrations between different pairs of methods for samples where turbidity was noted to be present in one or more methods. The unpaired t test was used to compare log transformed paraprotein concentrations between turbid samples and nonturbid samples. Correlations between phosphate and paraprotein concentrations in the turbid and non-turbid groups were assessed using log transformed data. x2 analysis was used to test the association between paraprotein type and turbidity.

We read the article by Hawkins! with great interest and would like to report our findings on the subject.Serum phosphate concentrations were measured in 41 patients who have multiple myeloma with identified serum paraproteins by four methods, the first three methods being performed on a Cobas Mira analyser. (1) Direct single step method using ammonium heptamolybdate in sulphuric acid (absorbance: 340 nm); (2) a modification of the molybdate method with the addition of albumin and formate-(absorbance: 340 nm); and (3) an enzymatic method in which phosphate ions react with inosine in the presence of nucleoside phosphorylase to give ribose-l-phosphate and hypoxanthine. Hypoxanthine is oxidized to xanthine, then to uric acid and the hydrogen peroxide formed measured as the quinoneimine product when it reacts with 4-aminophenazone (absorbance: 550 nm). (4) Technicon SMAC 3 (absorbance: 340 nm) by courtesy of Mr Ian Gibb, Department of Clinical Biochemistry, Royal Victoria Infirmary, Newcastle upon Tyne, UK. Paraprotein concentration was determined by integration of the electrophoretic peak area using a Corning 720 Scanning Densitometer on 33 samples.Spuriously high phosphate concentrations were obtained in the direct acid/molybdate method in 11 out of 41 samples due to the occurrence of turbidity in the reaction cuvettes. Turbidity, and hence spuriously high phosphate concentrations, was noted in two out of 41 samples by the albumin/formate method which had previously been reported by Bakker et al? to be free of this problem. The enzymatic method and the Technicon SMAC which incorporates a dialysis step were not susceptible to turbidity problems.We did not find any association between paraprotein type and the occurrence of turbidity by chi-squared analysis. The mean of 10gIO paraprotein concentrations in samples where turbidity was noted by methods 1 and/or 2, n = 11, was significantly higher than the mean of Ann Clin Biochem 1992; 29: 237-238 10gIO paraprotein concentrations in samples where turbidity was not observed in both methods 1 and 2, n=22 (P

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