Spurious hypocalcemia in patients undergoing maintenance hemodialysis.

Unusually low values of blood calcium were observed erratically following storage of predialysis blood samples from certain uremic patients treated with long-term hemodialysis. These values were found to be spuriously low, a result of either adsorption of calcium onto the storage tubes or nonhomogeneous distribution of calcium in the aliquot. Such spurious hypocalcemia was observed with storage of serum as well as plasma, although it was more common in the latter. In nonuremic controls and azotemic patients not treated with dialysis, no decrease in calcium was observed with storage. Although the cause of this in vitro change is unknown, reliable values of blood calcium could be obtained only when predialysis samples from patients undergoing dialysis were analyzed or diluted immediately.