Ionized calcium concentration in maintenance hemodialysis patients.

Since numerous formulae for "adjusted" total calcium and "calculated" ionized calcium are used in clinical practice, serum total and ionised calcium concentrations were measured in 20 hemodialysis patients with a wide range of serum total calcium and albumin concentrations. Patients were evaluated pre- and post-dialysis to document the effect of pH. Pre-dialysis total calcium varied from 1.70 to 3.17 mmol/L (mean 2.52 +/- 0.08) and a very close correlation between total and ionized calcium was found (r = 0.842; p less than 0.001) with 50.2% being in the ionized form. Dialysis did not alter this relationship despite a significant increase in pH (0.09; p less than 0.01). A normal volunteer group also demonstrated a similar correlation between total and ionized calcium and while the non-ionized calcium concentration was positively and significantly correlated to the serum albumin concentration (r = 0.629; p less than 0.01) this was not the case in the dialysis patients, although the serum albumin range was much greater (18 to 46 g/L). These results suggest that the percentage of total calcium in the ionized form in dialysis patients is not different from the normal population and that minor changes in pH and albumin may not be as important as has previously been believed. While direct measurement is preferable, halving of the total calcium is a simple prediction of the ionized fraction.