New reagent for colorimetric assay of calcium in serum

We report a new single-reagent colorimetric assay for the determination of calcium in serum. The assay is based on a novel chromogenic tetracarboxylic acid chelating agent. The method has several advantages over the o-cresolphthalein complexone (CPC) method, including improved linearity, 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 obtained with an Instrumentation Laboratory atomic absorption spectrophotometer. 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 reagents is 1 week vs 8 h for the CPC method.