Ionized calcium: whole blood, plasma or serum?

To determine an optimal specimen type to be used for measurements of ionized calcium (iCa) so that it applies properly to the reference interval. Also to determine the validity of the pH correction that is applied to iCa measurements.

A reference interval study of normal volunteers was performed using four sample types namely balanced heparin (BH) whole blood, lithium heparin (LH) whole blood, plasma and serum. The sample was treated in an anaerobic fashion and analyzed at 0 and 40 minutes after venipuncture. The effect of pH correction as well as analysis time after collection was also studied.

The mean iCa was the highest in BH-treated whole blood when measured immediately. However, it was slightly lower at 40 min after collection (p < 0.001). In contrast, there did not appear to be a significant difference in results when LH-treated whole blood was analyzed at 0 or 40 min. The reference interval for serum was similar to that of whole blood. The reference interval for plasma was dramatically lower than whole blood and plasma. The reference intervals for pH adjusted ionized calcium (iCa-adj) were dramatically lower than those from all specimens without adjustment. The reason for this was that the reference interval for pH in this study had a strong alkaline bias on one instrument and a strong acidic bias on the other.

The sample of choice for ionized calcium analysis appears to be whole blood with either BH or LH. For the LH specimen, there is no significant change over 40 min whereas there is significant change for the BH specimens (-0.030 mmol/L, p < 0.0001). iCa-adj should not be used unless (i) very strict attention is paid to standardization of both the calcium and the pH and (ii) there is a very good reason to believe that the patients' pH is normal at 7.4.