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

Abstract

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°C in 110 μl 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/1 within series, 0.017 mmol/1 between days. The mean value for cCa²⁺ (at pH 7.4) in serum for 53 healthy adults was 1.249 mmol/1 with a standard deviation of 0.036 mmol/1. cCa²⁺ in capillary blood at the actual pH from 20 healthy volunteers gave a mean value of 1.215 ± 0.047 mmol/1 (± 2 SD) with pH 7.428 ± 0.031 (± 2 SD). The slopes ΔlgcCa²⁺/ApH 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 semiautomatic combined Ca²⁺ - pH electrode system makes the measurement of ionized calcium for clinical use a reliable and accurate analysis.

Key-words:

• blood
• calcium analysis
• electrodes
• evaluation
• hydrogen ion concentration
• ions
• reference values