Spectrophotometric micro method for measurement of dialyzable calcium by use of cresolphthalein complexone and continuous-flow analysis.

We adapted a method for dialyzable calcium from fluorometric detection by use of calcein to a more specific spectrophotometric determination with cresolphthalein complexone. The reagents are available commercially and perform satisfactorily with respect to noise level, drift, stability, and sensitivity. Construction of the continuous-flow manifold with commercially available components (injection blocks, coils, and dialyzer) and the use of a 9-mm pathlength flow cell in an AutoAnalyzer I colorimeter have permitted a sampling rate of 70/h, and decreased the volume of serum required to 130 microL. A comparison of 71 sera analyzed by the present and the calcein method gave means of 1.39 (SD 0.14) and 1.39 (SD 0.13) mmol/L, respectively. The regression equation was: present method = 1.016 calcein--0.022 mmol/L (r = 0.97). The CV for the new method, as determined from 46 randomized duplicate analyses, was < 1%. The reference interval, as evaluated from results for 93 different individuals, was 1.26-1.43 mmol/L. We conclude that our methods is an improvement with respect to noise level, drift, specificity, detectability, and more general availability of instrumentation. Moreover, the smaller sample volume required makes possible the routine measurement of dialyzable calcium in pediatric samples.