Pseudohyperphosphatemia and dysproteinemia.

We report the case of a patient with IgG multiple myeloma and pseudohyperphosphatemia, and the case of another patient in whom unexplained hyperphosphatemia led to the diagnosis of monoclonal gammopathy. The pseudohyperphosphatemia was due to the interference of monoclonal immunoglobulins with the phosphomolybdate colorimetric assay for phosphorus determination widely in use with some automated systems. Ultrafiltration of these patients' serum samples resulted in normalization of the elevated phosphorus values. Knowledge of this phenomenon may obviate confusion, unnecessary testing, and expenditure. It may also provide clinicians with a clue as to the presence of a dysproteinemia.