Intracellular and extracellular blood magnesium fractions in hemodialysis patients; is the ionized fraction a measure of magnesium excess?

To establish the best measure for determining magnesium overload, we measured ionized and total magnesium in serum and mononuclear blood cells and total magnesium in erythrocytes in blood of 23 hemodialysis patients, known for their disturbed magnesium homeostasis. When comparing the mean magnesium values obtained in the patient population with those of a control population, all of these magnesium markers, including the biologically active fractions, were significantly (P < 0.05) increased. Because serum total magnesium was not increased in all dialysis patients studied, the population was divided into two groups, according to total serum magnesium > 1.0 mmol/L or less than that. Results in these two populations showed that ionized serum magnesium and ionized magnesium in mononuclear blood cells might give a better indication about the magnesium status of the tested patients than the currently used total serum magnesium data. However, neither of the two markers, especially ionized serum magnesium, was able to discriminate fully between normal magnesium homeostasis and magnesium excess. We therefore conclude that the two ionized magnesium markers offer minimal advantage for this discrimination, and that the total magnesium concentration in serum remains the measurement of choice.