The effect of temperature on plasma ionized calcium measured in whole blood in vitro

Plasma calcium ion concentration (cCa2+) in samples from 50 healthy volunteers was measured at four temperatures (21, 26, 31 and 37 degrees C) using a calcium electrode based on a neutral carrier ligand. A small negative correlation was found between temperature and cCa2+, the coefficient being -0.0017 mmol . l-1 . degrees C-1 (P less than 0.001). The significance of this in clinical, physiological and instrumentation fields is discussed. The effect on measured cCa2+ of overnight storage at 4 degrees C was slight, but did increase the variance of the results. It is suggested that the most accurate results of cCa2+ will be obtained from fresh samples measured at body temperature.