Serum calcium status in health and disease: a comparison of measured and derived parameters

The relationship of serum ionised calcium to total calcium was investigated in three series of experiments, each using different ion-selective electrodes. In the first, total and ionised calcium were measured in healthy and patient groups to compare the predictive value of each estimation. In the second and third studies, measured ionised calcium was compared with ionised calcium calculated using 5 different formulae, and with total calcium, both uncorrected, and adjusted for varying protein content using eight formulae. In 144 of 149 healthy subjects, serum ionised calcium and total calcium were normal. There were discrepancies between serum ionised calcium and total calcium in 135 of 572 patients with conditions associated with abnormal calcium metabolism. Correction of total calcium, or calculation of ionised calcium did not significantly improve this figure. Thus, corrected or derived calcium values will not substitute for ionised calcium determination in patients with abnormal calcium metabolism.