PSEUDOHYPERPHOSPHATAEMIA INCIDENCE IN AN AUTOMATIC ANALYZER

A routine blood analysis, using the Hitachi 717 analyser, of an asympt omatic patient with multiple myeloma revealed a phosphate concentratio n of 6.2 mmol/l (reference range 0.8-1.4 mmol/l). There was no clinica l or biochemical evidence for secondary hyperphosphataemia. Two additi onal myeloma patients with a normal renal function were found to have serum phosphate concentrations of 5 and 4.7 mmol/l. Globulin-depleted sera of these patients were found to have phosphate levels within the normal range as assayed by a Hitachi 717. All these patients were foun d to have normal inorganic phosphate levels when a SMAC autoanalyser w as used, thus indicating spurious readings by the Hitachi 717. The inc idence of pseudohyperphosphataemia in 298 patients with normal renal f unction and hyperglobulinaemia was 8%. To test the direct effect of gl obulin on phosphate analysis by the Hitachi 717, globulin was precipit ated from serum of myeloma and non-myeloma patients by ammonium sulpha te. The analysed data showed a positive correlation between globulin c oncentration and the spurious phosphate levels. Furthermore, even when inorganic phosphate was completely removed from the tested samples, s purious phosphate readings were detected in the presence of globulins from either myeloma or non-myeloma patients by Hitachi 717. It can be concluded that the ammonium molybdate method for determining inorganic phosphate in the Hitachi 717 gives spuriously high phosphate levels i n the presence of a high serum globulin concentration.