Factors affecting the determination of ionized calcium in blood.

Reproducibility with the same electrode on the same day is expressed by the CV. For standard solutions CV = 0.68%, for blood CV = 1.36%, for serum CV = 0.92%. The measure with 2 different Orion electrodes performed on blood samples on the same day gives no more variation between electrodes than within electrode. Results of Ca++ measured with SS-20 Orion and ICA1 Radiometer electrodes on the samples of normal blood or serum are significantly higher with the ICA1 electrode than with the SS-20 Orion. The use of decreasing concentration of NaCl (170 to 110 mmol/l) in calibration solutions shows an apparent increase of Ca++ concentration in normal blood. Sampling conditions may affect Ca++ results. The pH of peripheric venous blood drawn without torniquet in 57 normal people was 7.34 +/- 0.03 and differs from the normal arterial pH values (740 +/- 0.02). By applying the following equation: Ca++ (pH 7.40) = Ca++ [1-0.53 (7.40 - pH)], Ca++ can be adjusted to pH 7.40. The correction in pathologic condition should, however, take in account the real arterial pH of the subject. We tested if sitting versus recumbent position could modify Ca++ results. The difference was non significant. Heparin, necessary for blood and plasma sampling, may affect the electrode membrane and binds calcium ions. We compared in the same blood sampling the same dilutions of 2 kinds of heparin. Heparin Radiometer: 10 microliters in 1 ml blood (8 UI) and heparin Roche: 10 microliters in 1 ml blood (50 UI) and observed significantly higher Ca++ results with the lower concentration of heparin. Storage of serum 24 hours at -20 degrees C does not alter the Ca++ results. After 7 days at -20 degrees C the Ca++ decreases by 3.6% in average, mainly due to pH increase.