Measurementof Free Calcium Ion in CapillaryBloodand 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 #{176}C.A sample volume of 50 tl 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 preci-sion 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. Addftional Keyplirases: ion-selective electrndes. elec