Hyperphosphatemia: A Clue to Diagnosis

Serum phosphate level reference range in adults is 2.5 mg/dL to 4.5 mg/dL and in children is 3 mg/dL to 6 mg/dL. The causes of hyperphosphatemia fall into four categories. These are decreased renal excretion of phosphorus, exogenous phosphorus administration, redistribution of phosphorus, and pseudohyperphosphatemia. We report a 69-year-old gentleman presented with the history of swelling of feet and facial puffiness of 1 month duration. He had renal failure with normal sized kidneys. Serum phosphorus was high. Advanced investigations revealed plasma cell proliferative disorder (clonal bone marrow plasma cells >10%) on bone marrow examination, presence of M band at the junction of beta-2 and gamma region, and elevated serum IgG and serum beta-2 microglobulin. Hyperphosphatemia in multiple myeloma may be true, or pseudohyperphosphatemia. Diligent history, examination, and investigations have yielded the possibility of pseudohyperphosphatemia owing to multiple myeloma in our patient. The interference with the phosphomolybdate ultraviolet assay for serum phosphorus estimation is one of the reasons of pseudohyperphosphatemia in multiple myeloma. The other mechanism of pseudohyperphosphatemia could be the direct binding of paraprotein to phosphorus.