Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1066 page)

   A 24-hour urine level <25 mg suggests Mg deficiency (in the absence of conditions or agents that promote magnesium excretion). If caused by renal loss, urine Mg should be >3.65–6 mg/day.

   If level is <2.4 mg/day, collect a 24-hour urine sample during IV administration of 72 mg of MgCl
₂
. Some 60–80% of the load is excreted by patients with normal Mg stores; <50% excretion suggests nonrenal Mg depletion.

   Limitations

   Serum magnesium levels may remain normal even when total body stores of magnesium are depleted up to 20%.

   Phylate, fatty acids, and an excess of phosphate impair Mg absorption

   Hemolysis yields elevated results because levels in erythrocytes are two to three times higher than in serum.

Suggested Reading

Lum G. Clinical utility of magnesium measurement.
Lab Med.
2004;35:106.

MAGNESIUM, URINE

   Definition

   Magnesium is an important but commonly neglected electrolyte. Magnesium deficiency is often inadequately documented by serum magnesium levels. Urinary magnesium analyses have been advocated before and after therapeutic magnesium administration to further investigate the significance of apparent low serum magnesium. Abnormal levels of magnesium are most frequently seen in conditions or diseases that cause impaired or excessive excretion of magnesium by the kidneys or that cause impaired absorption in the intestines. Magnesium levels may be checked as part of an evaluation of the severity of kidney problems and/or of uncontrolled diabetes and may help in the diagnosis of GI disorders. Renal magnesium wasting occurs in renal transplant recipients who are on cyclosporine and prednisone. Renal conservation of magnesium is diminished by hypercalciuria, salt-losing conditions, and the SIADH.

   
Normal range:

   Twenty-four–hour urine: 72–120 mg/day

   Random urine:

   Male: 18–110 mg/g creatinine