Hyperphosphatemia in multiple myeloma due to a phosphate‐binding immunoglobulin

Hyperphosphatemia (HP) is usually seen in patients with hypoparathyroidism, renal failure, and tumor lysis. The authors described a patient with HP due to a phosphate‐binding immunoglobulin (Ig). An 86‐year‐old woman had serum phosphate levels as high as 4.75 mmol/l, (normal, 0.77 to 1.45 mmol/l). Serum ionized calcium, blood urea nitrogen (BUN), creatinine, and N‐terminal parathyroid hormone (PTH) levels were normal, but serum 1,25‐dihydroxyvitamin D level was subnormal at less than 12 pmol/l (normal, 36 to 146 pmol/l). Serum total protein was elevated at 105 g/l (normal, 60 to 80 g/l), and additional studies confirmed a diagnosis of immunoglobulin G (IgG) multiple myeloma. Results of in vitro studies using anti‐human IgG antibodies showed that the IgG of the patient bound inorganic phosphate. Several isolated case reports have documented spurious HP due to interference of the paraprotein in the routine serum phosphate assay. In only one patient, however, has actual binding of phosphate to a myeloma protein been documented. The studies of the authors document phosphate binding by an IgG paraprotein and suggest that in this setting HP may be of physiologic significance as evidenced by depressed serum levels of 1,25‐dihydroxyvitamin D.