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We compared physiologically abnormal low and high ionized magnesium (iMg) results determined with the AVL 988-4 (AVL, Graz, Austria) and Nova CRT (Nova Biomedical, Waltham, Mass) ion-selective electrodes (ISEs) in serum samples from randomly selected patients. A result of < 0.39 mmol/L with either ISE constituted the low magnesium group and of > or = 0.65 mmol/L the high magnesium group. Within each group we found significant differences between the iMg results. Major intermethod differences were found for samples with physiologically normal total magnesium concentration: most of the samples in the low magnesium group (83%) had abnormally low results with the Nova ISE, whereas most of the results with the AVL ISE (83%) were normal. In contrast, all results with the AVL ISE for the high magnesium group were abnormally high, but 67% of the results with the Nova ISE were normal. The agreement for the clinical interpretation of iMg results based on the reference interval for each method was only 32%. The differences in iMg results between the two analyzers must be resolved before using the iMg test as measured with ISE for patient care.