Ca2+ measurement with ion selective electrodes. The French coordinated evaluation of seven analyzers, for a better clinical relevance and acceptance

The measurement of ionized calcium has evolved in the last decade, and can now be easily performed by clinical laboratories using direct potentiometric analyzers available from a number of manufacturers. An original protocol for a comparison of the analyzers was used through a parallel multicenter evaluation in France. Using newly developed aqueous buffered solutions, this study focused not only on the analytical performance and operational handling of analyzers, but also on possible interferences in biological samples and the clinical relevance of the measurement with respect to the techniques of sample collection. All the instruments exhibited good precision and linearity, and were easy to handle and robust for daily use. However, not all the models gave identical results on the same patient's specimen. The utility of some Ca2+ analyzers has been further enhanced by the ability to "correct" the results to pH 7.40, although care must be taken in the interpretation of these results. While there are a number of clear-cut situations in which Ca2+ measurement is more relevant than total calcium, it seems that chemical activity of Ca2+ in blood may sometimes be considered with great caution under pathological conditions. The role of Ca2+ measurement in routine will be discussed in relation to the potential benefits of the instruments in laboratories.