Sampling and storage of blood for determination of ionized calcium

Abstract

We report some new procedures for the determination of ionized calcium using a semi-automatic ICA 1 analyser provided with a new flow-through tonometer unit, TNC 1, in connection with an automatic turntable. Ionized calcium calculated to a standard pH of 7.4 (c_Ca²⁺(7.4)) was not significantly different for capillary blood, venous blood and serum in healthy adults; however, a significant difference was found for actual ionized calcium (c_Ca²⁺). The effect of erythrocytes on the liquid junction potential was eliminated by use of a new salt bridge solution (sodium formate). Serum samples should be taken as anaerobi-cally as possible to avoid precipitation of calcium complexes when pH increases above 7.9. For measurement of c_Ca²⁺ + -pH we developed special capillary tubes. Serum could be stored at 4 d`C for 7 days and at -20 d`C for 45 days without significant change in c_Ca²⁺ +(7.4). Capillary blood could be stored at 4 d`C for 4 h without significant change in + and pH or at 4 d`C or 2 0 d`C for 24 h without significant change in c_Ca²⁺(7.4). Vein puncture with stasis or muscular exercise caused only a small increase in c_Ca²⁺(7.4) (1.5%). The ingestion of food did not affect c_Ca²⁺. in healthy adults. 97.5% CO₂ equilibrium was obtained in 3 min with the new flow-through tonometer.

Key Words:

• aerobic/anaerobic
• carbon dioxide
• diet
• hydrogen ion concentration
• instruments
• ion-selective electrodes
• laboratory
• new tonometer (TNC1)