Determination of serum ionised calcium by ion-exchange electrode in normal subjects.

Serum ionised calcium concentration [Ca2+] was measured with a calcium selective electrode in 65 normal people. A mean value of 1.06 mmol/l (+/- 0.04 S.D.) with an actual range of 0.97 to 1.13 mmol/l was obtained. Serum samples refrigerated at 4 degrees C for 24 h were satisfactory for analysis. Storage of whole blood samples for 6 h at room temperature before separation caused a very small error (+0.04 mmol/l). Mean results and ranges were similar in males and females and there was no significant correlation with age. No significant change in serum [Ca2+] was found following a normal meal. Serum [Ca2+] and total serum calcium showed a very slight correlation (r = 0.35). The method is reproducible and sensitive.