Use of the reference method for determination of serum calcium in a quality-assurance survey

The Reference Method [Clin. Chem. 19, 1208 (1973)] for determination of total calcium in serum has been applied to 29 specimens from the Wellcome Group Quality-Control Programme. The accuracy of the mean results for all participants, and for selected groups of participants, was determined over the concentration range 2.00-2.75 mmol/liter. We show that, in general, continuous-flow (AutoAnalyzer) and atomic absorption methods of analysis have a small, constant bias, but chelation (ethylenediaminetetraacetate) methods are unpredictably inaccurate.