Serum ionized magnesium in dialysis patients

Abstract

Background: Circulating magnesium exists in an ionized state and in an undissociated form, either bound to proteins or complexed to various anions, and only the fraction of free ionized magnesium (iMg) is biologically active. Methods: We measured serum iMg in 29 hemodialysis (HD) patients and 24 peritoneal dialysis (CAPD) patients using an ion-selective electrode. The results were compared with those from 30 age-matched control patients who were admitted due to non-renal diseases and were confirmed to have normal serum total magnesium (tMg) levels. Results: Serum tMg was significantly higher in both HD (0.95 [0.81 - 1.06] mmol/L) and CAPD (0.86 [0.74 - 0.99] mmol/L) patients compared to the controls (0.74 [0.70 - 0.78] mmol/L). Serum iMg was significantly higher in HD patients (0.65 [0.60 - 0.72] mmol/L) compared to the controls (0.55 [0.51 - 0.57] mmol/L), but was not higher in CAPD patients (0.54 [0.50 - 0.60] mmol/L). Thus, the ionized fraction of serum magnesium (iMg/tMg) was significantly lower in CAPD patients (65.3 [58.8 - 68.2]%), but not in HD patients (68.7 [63.6 - 77.0]%), compared to the controls (72.2 [67.2 - 78.3]%). In dialysis patients, iMg/tMg was significantly lower in those with residual renal function than in anuric patients (67.5 [63.6 - 74.8]% versus 61.4 [56.4 - 70.7]%), and anuria was present significantly more often in HD patients than in CAPD patients (27/29 versus 12/24). The ionized fraction of serum magnesium correlated with neither serum protein nor albumin, whereas the ionized fraction of serum calcium correlated significantly with both serum protein and albumin. Conclusions: In current practice, true hypermagnesemia may exist frequently in HD patients (at predialysis), but not in CAPD patients. The ionized fraction of serum magnesium may be reduced in CAPD patients, but not in HD patients. These findings appear to result from differences in dialysis modality, dialysate magnesium concentration, and residual renal function, but not from differences in serum protein or albumin levels.