Paraprotein interference with colorimetry of phosphate in serum of some patients with multiple myeloma

In untreated serum of three patients with multiple myeloma, concentrations of inorganic phosphate ranged from 130 to 270 mg/L as measured with a chromogenic assay based on the interaction of phosphate ion with ammonium molybdate in the presence of ferrous sulfate. There were no clinical features of hyperphosphatemia, and values for total calcium concentration in serum remained within normal limits throughout. Subsequent investigations demonstrated that this hyperphosphatemia was spurious and was caused by high concentrations of the paraprotein interfering with the chromogenic assay. Because this type of assay, adapted for automated systems, is now widely used in clinical laboratories, we call attention to this limitation to avoid confusion in clinical evaluations of patients with multiple myeloma.