Falsely high ionized magnesium results by an ion-selective electrode method in severe hypomagnesemia

Changes in serum total and ionized magnesium (Mg and Mg2+) and calcium (Ca and Ca2+) were monitored in three patients who transiently developed severe (total Mg < 0.50 mmol/l) to profound hypomagnesemia (total Mg < 0.35 mmol/l) due to cisplatin or interleukin-2 therapies. Mg2+ and Ca2+ were measured with the Nova ion-selective electrodes at 37 degrees C and all results were normalized to pH 7.40. Independent of the etiology, the Mg2+ fraction (Mg2+/total Mg) increased as the concentration of the serum total Mg decreased in all three patients. When the total Mg was around or below 0.35 mmol/l the Mg2+ approached or exceeded total Mg, suggesting an error in the measurement of Mg2+. The findings were extended by including a group of 31 additional patients whose serum total Mg, Mg2+, total Ca, and Ca2+ concentrations varied from abnormally low to above normal. The serum total and ionized concentrations strongly correlated for both Mg (r2 = 0.88) and Ca (r2 = 0.92). The Mg2+ fraction rapidly increased with a fall in the total Mg concentration (r2 = 0.76) and total Mg/total Ca ratio (r2 = 0.71). In fact, with decreasing total Mg concentrations or total Mg/total Ca ratios, the Mg2+ fraction progressively increased to 93-128% of the total, confirming an error in the Mg2+ determinations. The Ca2+ fraction showed a slight and insignificant decrease with falling total Ca concentrations and total Mg/total Ca ratios. The Mg2+ concentration was directly related (r2 = 0.62), whereas the Ca2+ concentration showed a complex relationship to the total Mg/total Ca ratio. Whether this latter relationship represents a technical artifact or a true biological phenomenon requires further study. The apparent overestimation of Mg2+ at very low total Mg concentrations, and in the presence of a very low total Mg/total Ca ratio, could be due to improper chemometric correction of the Ca effect on the Mg electrode, non-linearity, and inadequate calibration. Whatever the mechanism, the failure of this method to correctly measure very low serum Mg2+ concentrations in the sera of patients with severe hypomagnesemia, or likely in any patient with an unusually low total Mg/total Ca ratio, erodes its diagnostic usefulness.