Measurement of serum ionized versus total levels of magnesium and calcium in hemodialysis patients.

Until recently, only techniques for measuring total magnesium have been available. Now commercially available instruments using new ion-selective electrodes (ISE) for Mg+2 have made possible reliable measurement of ionized magnesium also in clinical practice. We measured changes induced by a hemodialysis session in serum ionized and total pools of magnesium and calcium using ISE methods. When compared with levels in age- and sex-matched control subjects, both serum ionized magnesium (0.68 +/- 0.11 vs. 0.56 +/- 0.06 mmol/l, p < 0.001) and total magnesium (1.00 +/- 0.19 vs. 0.82 +/- 0.08 mmol/l, p < 0.001) were higher in hemodialysis patients. The fraction of ionized Mg was 68.6 +/- 2.9% in hemodialysis patients, and did not differ significantly from that in controls (68.7 +/- 5.3%). The postdialysis value was 68.1 +/- 7.7%. The corresponding ratios of calcium (ionized/total) were 51.0 +/- 2.8% pre- and 50.9 +/- 4.6% postdialysis. Both prior to and after dialysis the correlation between ionized and total magnesium was high (r = 0.976, p < 0.001, and r = 0.925, p < 0.001, respectively). The corresponding ionized versus total calcium correlations were r = 0.724 (p < 0.001) before and 0.423 (p = 0.003) after dialysis. The changes induced by a hemodialysis session in serum concentration of ionized magnesium and calcium were dependent on the concentration of the cation in the dialysate. The change in PTH (suppression or stimulation) was very closely related to the changes in the serum concentration of ionized calcium. We concluded that measurement of ionized magnesium using ion-selective electrodes for Mg++ is an interesting new method in evaluating body magnesium status. Its definitive role in clinical practice cannot be judged on the basis of the results of the present study, but it will probably not achieve the same importance as the measurement of ionized calcium in clinical nephrology.