Protein interferes with ionised calcium measurement at the reference electrode liquid junction.

The ionised calcium concentration of sequential retentates prepared by ultrafiltration of a human control serum increased with increasing protein concentration when measured with both a Nova 2 and a Radiometer ICA1 analyser using their standard reference electrodes. In contrast, the ionised calcium in the same retentates fell slightly with increasing protein when the reference electrode liquid junctions of the instruments were changed from hypertonic to isotonic solutions, the values then paralleling those in the filtrates. Thus, the clinically significant positive relationship between ionised calcium and protein that has been reported with the Nova 2 and ICA1 analysers is almost certainly an effect of protein on the reference electrode liquid junction potential rather than a consequence of a Donnan effect on true ionised calcium distribution.