Spurious hypophosphatemia in a patient with multiple myeloma.

We report a patient with multiple myeloma and a prolonged history of hypophosphatemia who had remained asymptomatic. Extensive evaluation for a cause, including the search for a renal tubular disorder, oncogenous osteomalacia, or a parathyroid hormone (PTH)-related protein was unproductive. Renal biopsy showed no evidence of myeloma kidney. Subsequent mixing of the immunoglobulin G (IgG) fraction from the patient's serum with normal human serum, confirmed that the observed hypophosphatemia was spurious, resulting from interference of large amounts of an abnormal IgG with a standard automated laboratory assay for phosphate.