Clinical validation of dialysable calcium in relation to other methods of serum calcium measurement.

Dialysable calcium (CaD) values were measured by a simple technique not interfered with by protein bound calcium and validation attempted by comparison with concentrations of ionised calcium (CaI) and clinical categorisation. CaD values were also compared with total calcium (CaT) and albumin adjusted calcium (CaA) concentrations. The normal ranges for CaD, CaT, CaA, and CaI were calculated from the results in healthy blood donors. In 50 normal subjects CaD was more highly correlated with CaI than CaT or CaA. The effects of lying down and of venous stasis in 10 normal subjects showed that CaD was slightly influenced by posture only, whereas CaT was noticeably affected by posture and venous stasis; CaA reduced but did not abolish these effects on CaT. The correlation coefficient of CaD and CaI in patients with chronic renal failure was 0.81. CaD was compared with CaT and CaA values in 293 consecutive hospital patients; discrepant results were obtained in 14.3% and 13.0% of cases respectively and there were some clinical grounds for accepting CaD as correct in 86% and 74% of these cases. Measurement of CaD is a simple, reliable method for estimating accurately the calcium concentration free from biologically inactive protein bound calcium.