| abstract |
A single, stable reagent and automatable methods utilizing the reagent for the assay of calcium in serum and aqueous samples, such as urine. (Aqueous samples contain little or no protein.) The reagent includes arsenazo III, 8-hydroxyquinoline, a buffer that does not bind calcium competitively with arsenazo III and that provides a reagent pH from about 8.5 to about 10, the buffer being in sufficient amount to maintain pH within +/- 0.1 pH units of the reagent pH when the reagent is added to a sample of unknown calcium content, dimethylsulfoxide and water. Importantly, dimethylsulfoxide permits the incorporation of 8-hydroxyquinoline into a single, stable reagent, which is at an alkaline pH that is optimal for calcium measurement by the calcium-arsenazo III complex. 8-Hydroxyquinoline eliminates assay interference due to magnesium ion. Also, at least one non-ionic surfactant in sufficient amount to minimize lipid turbidity may be added to the reagent. Spectral measurements of calcium-arsenazo III are made at wavelengths that minimize interference from lipemia, hemolysis, and bilirubinemia. Spectral interference from lipemia and hemolysis is further reduced by employing bichromatic measurements of the calcium-arsenazo III complex and by employing serum blank measurements. |