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Serum Ionized versus Total Magnesium in Patients with Chronic Renal Disease

Background: Recent technology has made it possible to assess serum ionized magnesium with user-friendly ion-selective electrodes similar to measurement of the serum calcium concentration. Methods: We measured the serum ionized (Mg2+) and total magnesium (tMg) concentration in 69 patients with chronic kidney disease (38 men, 31 women, age 22–85 years) and in 75 control subjects. Results: In control subjects the reference ranges were as follows: serum Mg2+ 0.45–0.67 mmol/l, and tMg 0.67–0.96 mmol/l. In patients with chronic renal failure serum Mg2+ was 0.57 ± 0.05 mmol/l and tMg 0.80 ± 0.11 mmol/l, and in transplant recipients receiving cyclosporine 0.53 ± 0.05 and 0.71 ± 0.10 mmol/l, respectively. The correlation coefficient between serum Mg2+ and tMg was r = 0.73 (p < 0.001) in the patient group and r = 0.75 (p<0.001) in control subjects. Serum tMg was below the reference range in 15 of 69 measurements. However, serum Mg2+ was below the reference range in only 1 of these 15 samples. Hence, 14 of 69 cases with low tMg but normal Mg2+ were false-positive with respect to hypomagnesemia. Conclusion: We conclude that tMg may overestimate the incidence of hypomagnesemia, and the measurement of Mg2+ may be of benefit when studying patients with expected hypomagnesemia.