Improvements in and clinical utility of a continuous-flow method for routine measurement of dialyzable (ultrafiltrable) calcium.

We describe modifications to the original continuous-flow procedure for dialyzable calcium (Clin. Chem. 23: 1258, 1977) needed to make the method more suitable for routine clinical laboratory use. The modifications simplify the continuous-flow (AutoAnalyzer) manifold, decrease baseline noise, increase the sensitivity, and permit use of a less-expensive fluorometer. Bias due to variation in serum processing is minimized by use of serum samples minimally exposed to air and a pH 7.40 buffer in place of the routinely processed sera and pH 7.30 buffer used formerly. Day-to-day precision (CV) during the past year for samples that included three different lots of quality-control sera was 2 to 3%. The analysis requires 200 micro L of serum, collected with minor additional precautions. We find that dialyzable calcium can be dependably measured in the routine service laboratory and show how this information is clinically more useful than is information on total calcium in serum.