EVALUATION OF A NEW SEMIAUTOMATIC ELECTRODE SYSTEM FOR SIMULTANEOUS MEASUREMENT OF IONIZED CALCIUM AND PH

  • 1 January 1982
    • journal article
    • research article
    • Vol. 42  (5) , 407-415
Abstract
The instrument (ICA 1 Radiometer, Copenhagen) measures the activity of Ca2+ and H+ and displays the estimated concentration of free Ca2+ (cCa2+) and pH at 37.degree. C in 110 .mu.l 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 .apprx. 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 Ca electrode showed a fall from 100% to 92% over 20 wk. The cell potential of the Ca cell showed a slow drift of -0.02 mV/day over a 20-wk period while the pH cell which employs the same reference electrode showed no drift. Interference of Na+, K+, Li+, Mg2+ and H+ on the Ca 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 SD 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 SD 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 (.+-. 2 SD) with pH 7.428 .+-. 0.031 (.+-. 2 SD). The slopes .DELTA.logcCa2+/.DELTA.pH measured after equilibration at 2 different PCO2 [CO2 tension] values in venous blood (n = 20) and in serum (n = 104) gave a mean value of -0.221 .+-. 0.040 (.+-. 2 SD). The semiautomatic combined Ca2+ - pH electrode system makes the measurement of ionized Ca for clinical use a reliable and accurate analysis.