Spurious hyponatremia in multiple myeloma

The association of excessive hyponatremia (118-127 mEq/l) and an anion gap of 9 is reported in a patient with multiple myeloma. Intracellular, extracellular and total body exchangeable sodium were all elevated. The artificial nature of the hyponatremia was proven by correction of the sodium concentration by plasmapheresis in vitro. Artifactual hyponatremia should be suspected in the presence of excessive hyperproteinemia, normal serum osmolality and absence of clinical symptoms of hyponatremia.