In Vivo and in Vitro Studies on Ionized Versus Total Serum Calcium in Hyperparathyroidism

Measurements of ionized (Ca+ + ) calcium and total (Cat) calcium have been performed in sera from hyperparathyroid, parathyroidectomized and control subjects. The relationship between Ca++ and Cat has been analysed statistically. It was found that two straight lines with the point of intersection at Ca++ =2.75 and Cat = 5.55 mEq./l, and slopes of 0.628 and 0.447, respectively, would describe the data more accurately than one single line. However, when the calcium level was varied by the addition of calcium chloride in vitro in individual or pooled sera, the Ca++–Cat relationship appeared to be linear. The results suggest that sera from hyperparathyroid subjects have an increased calcium binding capacity.