NEW REAGENT FOR COLORIMETRIC ASSAY OF CALCIUM IN SERUM

We report a new single-reagent colorimetric assay for the determinatio n of calcium in serum. The assay is based on a novel chromogenic tetra carboxylic acid chelating agent. The method has several advantages ove r the o-cresolphthalein complexone (CPC) method, including improved li nearity, better accuracy, increased stability, and lower working pH (9 .0). The calibration curve for the assay is linear up to 4 mmol/L. The results for 47 patients' samples correlated well with results obtaine d with an instrumentation Laboratory atomic absorption spectrophotomet er. The mean (+/- SD) calcium values for the new method were 2.27 +/- 0.06 mmol/L and 2.26 +/- 0.05 mmol/L for the comparison method. The on -system stability of the reagent is 1 week vs 8 h for the CPC method.