Validation of an enzymatic total magnesium determination based on activation of modified isocitrate dehydrogenase

We have evaluated an enzymatic assay for the determination of Mg in serum, based on the activation of a chemically modified form of isocitrate dehydrogenase by Mg2+. In the presence of potassium isocitrate and NADP+, NADPH, which absorbs light at 340 nm, is produced, and its increase is proportional to Mg2+ in the sample. We have shown linearity to at least 5 mmol/L Mg and good precision by within-run CVs between 1.4% and 3.5% and day to day CVs between 3.2% and 5.4%. The method (y) is accurate, comparing well with the xylidyl blue method (x), with a Deming regression of y = 0.930x - 0.063 (r = 0.98, S(y/x) = 0.019), and with atomic absorption spectrophotometry (x), with a Deming regression of y = 0.932x - 0.016 (r = 0.97, S(y/x) = 0.023), and shows no interference from common endogenous compounds in blood and various metal ions except for visibly hemolyzed samples. A rapid assay, with reagents that are stable for at least 30 days at 4 degrees C and requiring no prior preparation, it is simple to use and suitable for adaptation onto automated chemistry analyzers.