New rapid kit for determining calcium in serum and urine evaluated.

We describe the performance of a colorimetric reagent kit (Diagnostic Systems Laboratories, Inc., Webster, TX 77598) for measuring calcium directly in urine, serum, and ultrafiltered serum, and compare the results with those obtained with atomic absorption spectrophotometry. The CV for within-run precision was 2.8 and 2.1% for urine and whole serum, respectively (n = 10 each). Between-run precision for urine, whole serum, and ultrafiltered serum was 1.9, 1.6, and 2.2%, respectively (n = 8 to 10). Analytical recovery of added calcium from three different urine and serum specimens, to which three different concentrations of calcium had been added, was 101.9 (SD 0.3%) for urine and 100.9 (SD 0.2%) for serum. Assay of 30 urine specimens, 15 ultrafiltered serums, and 20 whole serums by both the kit and atomic absorption spectrophotometry demonstrated correlation coefficients of 0.993, 0.828, and 0.751, respectively. Mg2+, hemolysis, or lipemia does not interfere. Compared with atomic absorption spectrophotometry, the calcium kit procedure is rapid and simple.