Measurement of free calcium ion in capillary blood and serum

We describe a new calcium ion-selective electrode for measurement of the substance concentration of free calcium ion [Ca2+] in the plasma phase of whole blood and in serum at 37 degrees C. A sample volume of 50 microliter suffices to obtain simultaneous values of pH and [Ca2+]. We found the within-series analytical standard deviation for serum to be 0.013 mmol/litre (CV, 1.1%) and day-to-day precision to be 0.022 mmol/litre (CV, 1.7%). The reference interval for [Ca2+] (at pH 7.40) in serum was found to be 1.184 +/- 0.054 mmol/litre (2 SD) from measurements on sera from 121 healthy blood donors. Measurements on capillary blood from 29 healthy volunteers gave a mean (+/- 2 SD) value for [Ca2+] (at pH 7.40) of 1.22 +/- 0.072 mmol/litre.