How should one approach the diagnosis of hypomagnesemia?
For most cases of hypomagnesaemia, the etiology can be elicited from a thorough history—focusing on diet, bowel habits, medications, comorbidities, diabetic control, and alcohol intake.
Additional testing includes the fractional excretion of Mg (FeMg; Equation 77.1). The serum magnesium is multiplied by 0.7 to account for the 30% of serum magnesium that is protein bound and not filtered at the glomerulus.
Normal values were established in patients with preserved kidney function in the setting of normal and low serum magnesium levels. Typical levels of FEMg were provided by Elisaf in a study of 216 patients (142 with normal magnesium and 74 with low magnesium of mixed etiologies). There was good separation of kidney wasting from extrarenal wasting in the study.
Fractional Excretion of Magnesium, Interpretation
|Normal magnesium level||1.8%||0.5%–4%|
|Renal magnesium wasting||15%||4%–48%|
|Extrarenal magnesium wasting||1.4%||0.5%–2.7%|
A fractional excretion of magnesium over 4% is typical for kidney magnesium wasting, though some references say 3% and others 2%.
Twenty-four-hour urine for magnesium can also be done. Urine magnesium should be less than 24 mg in the absence of treatment and extrarenal magnesium loss. If there is a magnesium kidney loss, the urinary magnesium will be greater than 24 mg.