Comparative evaluation of two calcium ion-selective electrode systems, and their utility for monitoring steady-state changes in [Ca2+].

We compared ionized calcium concentrations ([Ca2+]) as measured with two ionized-calcium analyzers: the NOVA 2 and the Orion SS-20. Samples were obtained from 43 human volunteers, 213 patients, and five dogs (106 samples). In the [Ca2+] range of 0.85 to 1.8 mmol/L, [Ca2+] measurements in whole blood with the NOVA 2 consistently exceeded those measured with the Orion SS-20. However, in the normal range, this difference appeared to be smaller when we compared values for plasma or serum, and was absent over the entire range when we compared aqueous solutions. The normal human [Ca2+] in whole blood as measured with the NOVA 2 is 1.22 +/- 0.01 mmol/L (mean +/- SEM) and that with the Orion SS-20 is 1.12 +/- 0.01 mmol/L (p less than 0.0001 by paired t-test); the 95% confidence intervals were from 1.14 to 1.30 and from 1.02 to 1.22 mmol/L, respectively. Using dogs, we also tested the usefulness of the ionized-calcium electrode for monitoring [Ca2+] during infusion of either citrate or calcium chloride solutions, to produce steady-state alterations in [Ca2+] equilibrium. Frequent successive [Ca2+] measurements were essential to appropriately adjust the infusion rates of these solutions to achieve steady-state [Ca2+].