Biological and analytic components of variation in long-term studies of serum constituents in normal subjects. V. Estimated biological variations in ionized calcium.

Intraand interindividual components of variation in ionized calcium among normal individuals have been estimated. The basic data were means of duplicate analyses of total serum calcium, total serum protein, and serum albumin from 68 normal subjects, 10-12 weekly samples per person. The McLean-Hastings equation was used to estimate ICa2i. Use of observed albumin/globulin ratios, instead of an assumed constant, had negligible effect on mean [Ca2] or components of variation. The interindividual component of variation in [Ca 2+] was found to be the same as that in total calcium: 3%. Average intraindividual variation (0.045-0.05 mmol/Iiter), ap peared to be entirely attributable to analytical deviations in total protein and total calcium determinations. Thus, in the average normal individual, no physiologic variation in [Ca 2+] could be detected. Results agreed with recent data on [Ca2+1measured by calcium ion-selective electrodes.