[The ionized calcium fraction of the blood: dependence on different methodical factors]

In the present study the influence of different methodical factors (analyzer with ion-selective electrodes used, anticoagulation of blood samples with heparin, storing of samples at different temperatures) on the concentration of ionized calcium in blood was investigated. It could be shown that nowadays' systems for analysis of ionized calcium (ORION SS-20, AVL 980, NOVA 2 and 8, ICA 1) yield reproducible values for human as well as canine blood. System-related differences point to a varying degree of interferences on the ion-selective electrodes used. The concentration of ionized calcium is strongly dependent on different preanalytic factors. Heparin leads to a concentration-related reduction of calcium ions in blood. This negative impact of heparin can be overcome by substitution of the solvent for CaCl2 (2.5 mmol.l-1) without meaningful falsification of the measured values. At 4 degrees C blood samples can be stored without detectable changes in Ca++ concentration for 8 hours.