Free calcium in serum. II. Rigor of homeostatic control, correlations with total serum calcium, and review of data on patients with disturbed calcium metabolism.

Two hundred and fifteen measurements of free (ionic) and total calcium were performed on sera from 184 patients in whom some disturbance in calcium metabolism was suspected. The correlation Coefficient between free and total calcium was 0.94, but a scattergram indicated that free calcium concentration cannot be predicted from knowledge of total calcium concentration. In “normal” individuals the correlation coefficient was only 0.45. Homeostasis of free and total calcium was examined by making periodic measurements in seven normal individuals over a period of eight months; the variability around the individual’s mean value was 1.2% for free calcium and 1.9% for total calcium. Free calcium, as properly measured by the ion-specific electrode, is a better measure of calcium status than is total calcium in the disease states studied, which were mostly hyperparathyroidism, multiple myeloma, and renal failure.