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[Influence of impaired renal function and magnesium oxide administration on serum magnesium levels in elderly inpatients].

AIM
This study aimed to clarify whether serum magnesium (Mg) levels increased in elderly inpatients with impaired renal function receiving magnesium oxide (MgO) administration.


METHODS
We recruited a total of 1,282 inpatients (505 men, 777 women, mean age 79.6 years) in this study. Fasting blood samples were obtained early in the morning. Serum Mg was measured using xylidyl blue method. Estimated glomerular filtration rate (eGFR) levels were calculated according to the formula for ethnic Japanese, inserting sex, age and serum creatinine (cr) levels into the formula. Inpatients were divided into 5 groups according to eGFR levels (ml/min/1.73 m(2)): <30 eGFR (group 1), ≥ 30 but <60 (group 2), ≥ 60 but <90 (group 3), ≥ 90 but <130 (group 4), and ≥ 130 (group 5). Division into a further 4 groups was also carried out, into the same groups (1-3) as described above and ≥ 90 (group 4). In these subgroups we investigated how serum Mg levels changed according to different eGFR levels, or after being given MgO.


RESULTS
In 552 inpatients not given MgO and 372 given MgO, the percentages of subjects with ≥ 2.7 mg/dl of serum Mg were 38.5% in those not given MgO and 78.5% in those given MgO in group 1, 28.1% and 49%, respectively, in group 2, 0% and 23.1% to 29.6% in groups 3 to 5; the percentage of patients with < 2.4 mg/dl of serum Mg was higher in groups 1 to 5 in those not given MgO than in those given MgO. These findings suggest an increase in serum Mg levels after initiation of MgO administration. At an average of 6.9 months in 22 men and 6.4 months in 39 women, both groups not receiving MgO serum Mg increased significantly, while eGFR reduced considerably. At an average of 6.4 months in 18 men and 10 months in 30 women who received MgO, serum Mg increased considerably, although eGFR did not show any significant change. In 4 cases spanning 4 to 14 months, seesawing alterrations between eGFR and serum Mg were often noted. We measured subjects from the 4 subgroups (divided according to eGFR), comprising 88 inpatients not given MgO, 116 who were given daily doses of 0.5 g to 1.5 g MgO, and 118 who were given daily doses of 2 g to 3 g MgO. In those without MgO serum Mg was markedly higher in group 1 than in groups 3 and 4. In all 4 groups, serum Mg was markedly higher in those given MgO than in those not given MgO. In group 1 only, serum Mg was markedly higher in those given daily doses of 2 g to 3 g than in those given 0.5 g to 1.5 g MgO. In 23 subjects with serum Mg levels of over 3.8 mg/dl (normal range: 1.7 mg/dl to 2.6 mg/dl), 7 not given MgO had markedly lower eGFR levels than 16 given MgO, and the mean levels of serum Mg were similar among these. The highest levels of serum Mg were 5.2 mg/dl in those not given MgO and 5.9 mg/dl in those given MgO.


CONCLUSION
The important factors associated with elevated serum Mg levels noted in this study were: a reduction in eGFR to below 30 ml/min/1.73 m(2), and MgO administration for treatment of chronic constipation and the simultaneous occurrence of the above two factors.