Pseudohyperphosphataemia incidence in an automatic analyzer.

A routine blood analysis, using the Hitachi 717 analyser, of an asymptomatic patient with multiple myeloma revealed a phosphate concentration of 6.2 mmol/l (reference range 0.8-1.4 mmol/l). There was no clinical or biochemical evidence for secondary hyperphosphataemia. Two additional 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 found to have normal inorganic phosphate levels when a SMAC autoanalyser was used, thus indicating spurious readings by the Hitachi 717. The incidence of pseudohyperphosphataemia in 298 patients with normal renal function and hyperglobulinaemia was 8%. To test the direct effect of globulin on phosphate analysis by the Hitachi 717, globulin was precipitated from serum of myeloma and non-myeloma patients by ammonium sulphate. The analysed data showed a positive correlation between globulin concentration and the spurious phosphate levels. Furthermore, even when inorganic phosphate was completely removed from the tested samples, spurious 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 in the presence of a high serum globulin concentration.