Background: Circulating magnesium exists in the bound and in the free ionized form, that is biologically active. In kidney disease the relationship between ionized and total circulating magnesium is often altered. Little information is available on the influence of hemodialysis on the relationship between ionized and total circulating magnesium in end-stage kidney disease. Methods: Plasma total and ionized magnesium and the plasma ionized magnesium fraction were assessed before and after hemodialysis (dialysate magnesium content 0.75 mmol/l) in 46 patients with end-stage kidney disease and in a control group of 25 healthy subjects. Results: In patients plasma total (from 1.19 [1.05–1.33] to 1.10 [1.02–1.16] mmol/l; median and interquartile range) and ionized (from 0.71 [0.66–0.78] to 0.65 [0.63–0.69] mmol/l) magnesium significantly decreased during dialysis (control subjects: 0.82 [0.80–0.92], respectively, 0.57 [0.54–0.59] mmol/l). The plasma ionized magnesium fraction was significantly lower in patients both before (0.61 [0.58–0.64)] and after (0.60 [0.56–0.62]) hemodialysis than in controls (0.68 [0.65–0.70]). Conclusion: The study demonstrates a tendency towards a reduced circulating ionized magnesium fraction in end-stage kidney disease that is not corrected by hemodialysis.

Background: Circulating magnesium exists in the ionized state and in the undissociated form, either bound to albumin, or complexed to various anions. Until recently, only the measurement of total plasma magnesium has been possible. Now circulating ionized magnesium can be assessed as well. Methods: Total and ionized plasma magnesium were determined in 43 patients on maintenance hemodialysis (dialysate composition: calcium 1.75 mmol/l, magnesium 0.75 mmol/l) before a dialysis session and in a group of 23 healthy subjects. Results: The total (1.16 [1.03–1.31] versus 0.81 [0.78–0.89] mmol/l; median and interquartile range) and the ionized (0.71 [0.66–0.78] versus 0.54 [0.53–0.59] mmol/l) plasma magnesium levels were significantly higher (p < 0.01) and the ionized plasma magnesium fraction lower (0.61 [0.58–0.65] versus 0.67 [0.64–0.70]; p < 0.02) in patients than in controls. Conclusion: The determination of circulating ionized magnesium using selective electrodes is an attractive method to evaluate extracellular magnesium in kidney disease.