An automated method for serum calcium utilizing ethylene diamine tetraacetic acid.

A FUNDAMENTAL DIFFICULTY in estimating calcium in serum by photometry arises from the fact that the normal range is narrow relative to the concentration of calcium. In the cases of sodium and chloride which also possess narrow normal ranges, the automation problem has been approached in two different ways. For sodium the Technicon Flame Photometer incorporates an electrical range expander. In the Technicon N3a methodology for serum chloride the reagent contains mercuric nitrate which removes a certain amount of chloride from the reaction so that the calibration curve does not pass through the origin. In the present method, a similar effect is achieved for serum calcium by the addition of ethylene diamine tetraacetic acid (EDTA) to one of the reagents. Glyoxal-bis-(2-hydroxyanil) (GBHA) was used by Skerry in the estimation of calcium in serum by titration with EDTA (1). Mager and Farese (2) used it in the direct photometric analysis of serum calcium, a method which was adapted to the AutoAnalyzer by Farese et al (3). In the work reported here, a number of parameters have been studied. The method has been modified by the introduction of a dialysis step and by the use of an increased concentration of GBHA. A modified solvent is employed and EDTA has been incorporated into the buffer. GBHA