Evaluation of a new automatic calcium ion analyzer.

We evaluated a new automatic analyzer for ionized calcium (Ca2+), the Orion Model SS-20, based on a flowthrough ion-exchange electrode. Ca2+ was measured in heparinized whole blood and serum. Within-day variation was 1.2%, day-to-day variation1.6%, and analytical recovery 92.4%. Over the phsiological range interference by K+ and Mg2+ was negligible; major changes in ionic strength, induced by changes in N+ concentration, made correction for a sodium error necessary. Within the physiological range, Ca2+ was inversely correlated to variations in pH. Therefore, to compare Ca2+ values, correction to an apparent pH of 7.40 should be made. The calcium binding effect of heparin was negligible when minimal (4.4 int. units/ml) concentrations of heparin were used. Storage of serum at 4 degrees C for one week resulted in a 4 degrees decrease in apparent serum Ca2+, primarily owing to an increase in pH during storage. In normal material, mean values for blood-and serum-Ca2+(1.10 and 1.07 mmol/liter, respectively) were close to results obtained by previous systems. Errors caused by disturbances in the fluid flow and non-function of half the electrodes we received were the major inconveniences of the analyzer. We conclude that this new analyzer gives decisive advantages in measurement of Ca2+, making this importnant analysis possible as a routine laboratory test for the first time.