Artifactual hypercalcemia in multiple myeloma.

A calcium-binding IgG K monoclonal protein in a patient with multiple myeloma and asymptomatic hypercalcemia was recognized, isolated, and characterized. In addition to binding by the whole IgG molecules, calcium was bound by purified Fab fragments and recombined heavy and light chains. In a competitive binding study, the isolated myeloma protein did not bind magnesium. Recognition of calcium-binding myeloma proteins is important in order to avoid therapy for hypercalcemia.