The relationship of serum ionized and total calcium in primary hyperparathyroidism

Ionized and total calcium concentrations were measured in the same serum samples of 48 patients with primary hyperparathyroidism in order to compare diagnostic accuracy and sensitivity and to determine the intra-individual variation of these parameters in this disorder. In 149 samples, 0.7 per cent of serum ionized calcium and 7.4 per cent of total calcium determinations were within the normal range (mean +/- 2 S.D.). However, all patients exhibited abnormally elevated values upon repeated testing. Ionized and total calciums were significantly correlated both preoperatively (r=0.85 and p less than 0.001) and postoperatively (r greater than 0.9). Heterogeneous and abnormally large variations of both measurements were noted in these patients. It is concluded that the superior precision of the method for ionized calcium determination renders it more accurate than total calcium for the diagnosis of primary hyperparathyroidism. The disturbance of calcium homeostasis is reflected in both calcium and its ionic fraction in serum.