Arsenazo III: an improvement of the routine calcium determination in serum.

The widely used calcium dye reagent, o-cresolphthalein complexone, was replaced by the less well known metallochromic indicator Arsenazo III in a selective multitest analyser. Calibration stability increased from 12 hours to more than 48 hours. With the Arsenazo III reagent we clearly improved the within-lab and between-lab imprecision of the calcium determination. The accuracy of the Arsenazo III method was established with a reference method (atomic absorption spectrophotometry) and good results were obtained with assayed control sera. Furthermore, the mean calcium values of our out-clinic patients remained constant during the change of reagent. The calcium values are influenced by haemolysis; we correct for the observed decrease by adding 0.015 mmol/l Ca per 10 mumol/l Hb. Furthermore they are influenced by very small amounts of Cu ions from the preceding protein determination in the same cuvette. This latter problem is easily overcome by adding 15 mmol/l thiourea to the calcium reagent.