Comparison of hypertonic and isotonic reference electrode junctions for measuring ionized calcium in whole blood: a clinical study.

We measured ionized calcium concentrations in whole blood from 91 patients who had no clinical or biochemical evidence of disturbed calcium homeostasis and who had a wide range of serum albumin concentrations. We used both a standard Ciba-Corning 634 analyzer, which has a membrane-restricted saturated KCl reference electrode bridge, and a modified instrument with a 150 mmol/L NaCl bridge. After adjusting the externally standardized values from each instrument for their least-squares regressions on pH, there was a significant correlation between ionized calcium and albumin only with the standard analyzer. In contrast, only values from the modified instrument correlated with serum chloride; this was not explained by ionic strength or organic anion interferences. We conclude that there is unlikely to be any major advantage in using a membrane-restricted isotonic NaCl reference electrode for in vitro clinical measurements, although it may be of value for in vivo monitoring. The importance of measuring serum albumin when using most commercial ionized calcium analyzers is emphasized.