Determination of Serum Calcium by Use of an AutoAnalyzer I-li Hybrid

A hybridized system is described for the continuous-flow determination of serum calcium. The analysis is based on the cresolphthalein colorimetric procedure as described for use with the SMA 12/60 (Technicon Corp.). The hydraulic features of the AutoAnalyzer II are used with modules of the AutoAnalyzer 1. This combination gives both the ease of operation and hy-draulic improvements inherent in the AutoAnalyzer II system and provides an emergency alternative to the SMA 12/60, with which results are shown to be comparable. In selecting a back-up procedure for the SMA 12/ 601 (Technicon Instruments Corp., Tarrytown, N. Y. 10591), we wished to institute a method that was ac-curate, reproducible, and that gave results agreeing closely with those from the SMA 12/60. Results of the cresolphthalein procedure used with the SMA 12/60 (1, 2) agree well (3) with the results from atomic absorption (4), as we were able to confirm. The method of Pybus et al. (4a) has now been ac-cepted as the “referee method ” for accurate serum calcium estimation in a study completed by the U. S. National Bureau of Standards (4b). The accuracy of the method was established by comparison with isotope dilution techniques with use of isotope dilu-tion mass spectroscopy. Quality Control surveys car-ried out by the American College of Pathologists show the SMA 12/60 to be the most reproducible method reported (5). Routine quality control in this laboratory showed the coefficient of variation (CV) for results from the SMA 12/60 to range between 1.39 % and 2.26 % (n = 60) over a two-year period. For these reasons, we decided to use a cresolphth-alein back-up method. As wealso wished to use AAI modules already at hand in our laboratory, we adopted the following plan