Actual ionized calcium and pH in blood collected in capillary or evacuated tubes

Results are presented for the concentration of actual ionized calcium (cCa2+), the actual pH and ionized calcium at pH = 7.4 [cCa2+(7.4)] measured in capillary whole blood, venous serum from vacuum tubes (Venoject) and serum equilibrated to pCO2 approximately equal to 5.3 kPa from 57 renal patients. The mean value for cCa2+ was higher for venous serum than for capillary blood (1.29 mmol/l versus 1.26 mmol/l), corresponding to the difference in pH (in venous serum). This difference could be eliminated by separating the serum at 4 degrees C rather than room temperature. The cCa2+ (7.4) was the same for capillary blood, venous serum (Venoject) and equilibrated serum from blood collected in plain glass tubes, Venous serum could be stored at 4 degrees C for 4 h without significant change in cCa2+ and pH, and for 1 week at 4 degrees C for the measurement of cCa2+(7.4). In patients with acid-base disturbance cCa2+ is preferable to the measurement of cCa2+(7.4).