OBJECTIVE: To evaluate the magnesium status of continuous ambulatory peritoneal dialysis (CAPD) patients using a new method for assessing the level of the ionized fraction of serum magnesium. DESIGN: Serum ionized magnesium was measured in CAPD patients using the ion-selective electrode for Mg2+. SETTING: The Dialysis Unit of Tampere University Hospital. PATIENTS: Twenty-six patients on CAPD (age: 21-81 years, mean 54 +/- 16 years; duration of CAPD: 3-52 months, mean 13 months), and 26 sex- and age-matched healthy controls. RESULTS: Both serum ionized magnesium (0.73 +/- 0.11 mmol/L vs 0.56 +/- 0.07 mmol/L, p < 0.001) and total magnesium (1.11 +/- 0.22 vs 0.81 +/- 0.08 mmol/L, p < 0.01) were higher in CAPD patients than in sex- and age-matched controls. The ionized magnesium fraction of total magnesium was slightly lower in dialysis patients in spite of the fact that 16/26 patients had serum albumin less than 36 g/L. Hypermagnesemia (mean serum ionized magnesium 0.78 +/- 0.10 mmol/L) was observed in the 13 of 26 patients with 0.75 mmol/L Mg2+ dialysate; those with lower magnesium dialysate (Mg2+ 0.50 mmol/L in 10/26 and Mg2+ 0.25 mmol/L in 3/26) had mean serum ionized magnesium at the upper normal margin (0.69 +/- 0.10 mmol/L). CONCLUSION: In CAPD patients with Mg2+ 0.5-0.75 mmol/L in their dialysis fluid, both serum ionized and total magnesium concentrations were higher but the ionized/total magnesium ratio was lower than in healthy control subjects. Use of ion-selective electrodes to measure ionized magnesium may be a more useful methodology than measuring total magnesium in the evaluation of magnesium status of CAPD patients, because it is not influenced by hypoalbuminemia or increased complexed fraction of magnesium often present in dialysis patients.

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