Automated calmagite compleximetric measurement of magnesium in serum, with sequential addition of EDTA to eliminate endogenous interference.

We describe an automated method for magnesium assay, for use with the Cobas-Bio centrifugal analyzer. Magnesium is reacted with calmagite, a dye, and the absorbance of the magnesium-dye complex at 520 nm is measured. EDTA is then added to break up the complex and the absorbance at 520 nm is re-measured to correct for sample color and turbidity. Including ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) and triethanolamine in the dye reagent eliminates interference by calcium and iron. Within-run CVs were less than 2.0% for concentrations of magnesium ranging from 18 to 40 mg/L, and less than 4% for a magnesium concentration of 9.4 mg/L. Day-to-day precision data, determined over five months were: mean = 21.0 mg/L, CV = 2.7%; mean = 43.0 mg/L, CV = 3.2% (n = 550 for both). Comparison of the Cobas-Bio method (y) with an atomic absorption spectrometric method (x) gave the following results: y = 0.968x + 0.448, r = 0.986, Sy/x = 0.34 mg/L, mean x = 19.8 mg/L, mean y = 19.6 mg/L, n = 44. Hemoglobin, bilirubin, and turbidity do not interfere. The standard curve is linear up to a magnesium concentration of 97 mg/L.