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

In a two-center (Academic Medical Center, The Netherlands, and National Institutes of Health, USA) study, we compared ionized magnesium (iMg2+) results 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-selective electrode. The comparison was performed with frozen serum samples from normal individuals and patients. Imprecision and reference intervals were 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 samples from patients. With samples from normals, all three analyzers reported significantly different results (p<0.05). Best precision was found using the 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 high iMg2+, but 10.7 and 9.4%, respectively, at iMg2+ approximately 0.30 mmol/l. The reference intervals (mean+/-standard deviation) based on measurements in fresh serum samples were different for each analyzer: 0.55-0.63 mmol/l for AVL, 0.470.57 mmol/l for KONE and 0.43-0.55 mmol/l for NOVA. Thus, significant differences among the ionized magnesium concentration obtained with the three analyzers, limit comparison of results in clinical practice, and need to be resolved (e.g. by improvement of specificity and standardization of calibrators).