Serum ionized magnesium: Comparison of results obtained with three ion-selective analyzers

In a two-center (Academic Medical Center, The Netherlands, and National Ins titutes of Health, USA) study, we compared ionized magnesium (iMg(2+)) resu lts in serum determined with the AVL 988/4, KONE Microlyte 6 and NOVA CRT, which are the currently available analyzers equipped with a magnesium ion-s elective electrode. The comparison was performed with frozen serum samples from normal individuals and patients. Imprecision and reference intervals w ere established. We found the best agreement between the KONE(x) and AVL(y) magnesium ion-selective electrodes (y = 0.972x-0.013; n = 138) with sample s from patients. With samples from normals, all three analyzers reported si gnificantly different results (p < 0.05). Best precision was found using th e NOVA; coefficients of variation established at three levels were all <4.0 %. Coefficients of variation for the AVL and KONE were <5% at normal and hi gh iMg(2+), but 10.7 and 9.4%, respectively, at iMg(2+) approximate to 0.30 mmol/l. The reference intervals (mean +/- standard deviation) based on mea surements in fresh serum samples were different for each analyzer: 0.55-0.6 3 mmol/l for AVL, 0.470.57 mmol/l for KONE and 0.43-0.55 mmol/l for NOVA. T hus, significant differences among the ionized magnesium concentration obta ined with the three analyzers, limit comparison of results in clinical prac tice, and need to be resolved (e.g. by improvement of specificity and stand ardization of calibrators).