MEASUREMENT OF SERUM IONIZED VERSUS TOTAL LEVELS OF MAGNESIUM AND CALCIUM IN HEMODIALYSIS-PATIENTS

Until recently, only techniques for measuring total magnesium have bee n available. Now commercially available instruments using new ion-sele ctive electrodes (ISE) for Mg++ have made possible reliable measuremen t of ionized magnesium also in clinical practice. Wi: 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 he modialysis patients. The fraction of ionized Mg was 68.6 +/- 2.9% in h emodialysis patients, and did not differ significantly from that in co ntrols (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) b efore and 0.423 (p = 0.003) after dialysis. The changes induced by a h emodialysis session in serum concentration of ionized magnesium and ca lcium were dependent on the concentration of the cation in the dialysa te. The change in PTH (suppression or stimulation) was very closely re lated to the changes in the serum concentration of ionized calcium. We conclude that measurement of ionized magnesium using ion-selective el ectrodes for Mg++ is an interesting new method in evaluating body magn esium status. Tts definitive role in clinical practice cannot be judge d on the basis of the results of the present study, but it will probab ly not achieve the same importance as the measurement of ionized calci um in clinical nephrology.