Direct Measurement of Ionized Calcium (Standardization, Normal Values and Clinical Results)

Ionized calcium (Ca++) is the biologically active fraction of total serum calcium (Cat), thus it is of special clinical interest. In the diagnosis of primary hyperparathyroidism (pHPT) none of the previously reported methods1–5 for measuring (Ca)++ have become clinically useful because of cost. The development of a (Ca)++- selective membrane6 provided a simpler and more specific measurement of (Ca)++. The Orion SS-20 (R) made this technique available for clinical purposes. In order to evaluate this procedure we determined the factors that influence ionization7–10 and studied the usefulness11–14 of the procedure in the diagnosis of pHPT.