Centrifugal analysis with automated sequential reagent addition: measurement of serum calcium

• Published in: Clinical chemistry (Baltimore, Md.) Vol. 27; no. 12; pp. 2025 - 2028
• Main Authors: Liedtke, RJ, Kroon, G, Batjer, JD
• Format: Journal Article
• Language: English
• Published: England Am Assoc Clin Chem 01.12.1981
• Subjects: Autoanalysis - methods; Calcium - blood; Centrifugation; Chelating Agents; Edetic Acid; Humans; Nephelometry and Turbidimetry; Phenolphthaleins; Spectrophotometry; Spectrophotometry, Atomic
• ISSN: 0009-9147; 1530-8561
• DOI: 10.1093/clinchem/27.12.2025

We describe an automated method for calcium assay, for use with the Cobas-Bio centrifugal analyzer. Calcium is reacted with cresolphthalein complexone and the absorbance of the calcium--dye complex at 575 nm is measured. EDTA is then added to break up the calcium--dye complex and the absorbance at 575 nm is re-measured, to correct for endogenous color and turbidity. Day-to-day precision data, determined over four months, were as follows: mean = 92.9 mg/L, CV = 1.47%; n = 216; mean = 128.7 mg/L, CV = 1.72%; n = 216. Comparison of the Cobas-Bio method (y) with an atomic absorption spectrometric method (x) gave the following results: y = 1.012x--2.05, r = 0.991, Sy/x = 1.2, mean x = 92.63 mg/L, mean y = 91.69 mg/L, n = 74. Hemoglobin, bilirubin, or turbidity does not interfere. At the medical decision value (110 mg/L), the overall analytical error is 4.6 mg/L, which is less than the 5 mg/L allowable (95% confidence limit) error.