STATE-OF-THE-ART OF SERUM TOTAL CALCIUM MEASUREMENT AS INVESTIGATED BY SPLIT-SAMPLE MEASUREMENT WITH AN ION CHROMATOGRAPHY CANDIDATE REFERENCE METHOD

We evaluated bias and inaccuracy of four frequently used routine test systems for serum total calcium, using a candidate reference method ba sed on ion chromatography. The mean biases and 95% confidence interval s that we observed were 0.0 +/- 0.59% for Johnson & Johnson arsenazo ( III), 1.3 +/- 0.62% for Beekman arsenazo(III), -0.4 +/- 0.44% for Beek man applying ion selective electrode :measurement after sample dilutio n, and -1.9% +/- 0.42% for Boehringer o-cresolphthalein. The inaccurac y of all test systems was usually < 4.7% (calculated as deviation of s inglicates from ion chromatography). Both bias and inaccuracy are disc ussed in the light of specifications set by expert groups or derived f rom the biological variation of serum total calcium. The study reveale d that the intrinsic quality of commonly used test systems for serum t otal calcium satisfies even some of the more stringent criteria for me thod bias and inaccuracy.