Evaluation of a new semiautomatic electrode system for simultaneous measurement of ionized calcium and pH.

The instrument (ICA 1 Radiometer, Copenhagen) measures the activity of calcium ion and hydrogen ion and displays the estimated concentration of free calcium ion (cCa2+) and pH at 37 degrees C in 110 microliter of whole blood or serum. The cCa2+ at pH 7.40 is calculated by means of a fixed relationship between cCa2+ and pH. The results are indicated on a display approximately 1 min after sample introduction. The measuring cycle plus automatic rinsing takes 3 min. The apparatus is equipped with a number of control functions. The sensitivity of the calcium electrode showed a fall from 100% to 92% over 20 weeks. The cell potential of the calcium cell showed a slow drift of--0.02 mV per day over a 20-week period while the pH cell which employs the same reference electrode showed no drift. Interference of Na+, K+, Li+, Mg2+ and H+ on the calcium measurement in whole blood is estimated to be at most +0.1%. The effect of erythrocytes on the static liquid junction potential is minimized by a special extrapolation procedure. The analytical standard deviation for cCa2+ in serum was 0.008 mmol/l within series, 0.017 mmol/l between days. The mean value for cCa2+ (at pH 7.4) in serum for 53 healthy adults was 1.249 mmol/l with a standard deviation of 0.036 mmol/l. cCa2+ in capillary blood at the actual pH from 20 healthy volunteers gave a mean value of 1.215 +/- 0.047 mmol/l (+/- 2SD) with pH 7.428 +/- 0.031 (+/- 2 SD). The slopes delta lgcCa2+/delta pH measured after equilibration at two different PCO2 values in venous blood (n = 20) and in serum (n = 104) gave a mean value of -0.221 +/- 0.040 (+/- 2 SD). The semi-automatic combined Ca2+ - pH electrode system makes the measurement of ionized calcium for clinical use a reliable and accurate analysis.