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Magnesium - Deficiency Test

Measurement of urine magnesium is especially useful because magnesium deficiency is detectable in urine before it changes serum magnesium levels. This test may be used to rule out magnesium deficiency as the cause of neurologic symptoms and to help evaluate glomerular function in suspected renal disease.

Magnesium is a cation found primarily in the bones and in intracellular fluid; a small amount is present in extracellular fluid. This element activates many enzyme systems, helps transport sodium and potassium across cell membranes, affects nucleic acid and protein metabolism, and influences intracellular calcium levels through its effect on secretion of parathyroid hormone.

Purpose

• To rule out magnesium deficiency in patients with symptoms of central nervous system irritation
• To detect excessive urinary excretion of magnesium
• To help evaluate glomerular function in renal disease

Patient preparation

• Explain to the patient that this test determines urine magnesium levels.
• Tell him that this test requires urine collection over a 24-hour period.

Procedure and posttest care

• Collect the patient's urine over a 24hour period.

Precautions

• Tell the patient to be careful not to contaminate the urine specimen with toilet tissue or stool.

Reference values

Normal urinary excretion of magnesium is 6 to 10 mEq/24 hours or 3 to 5 mmol/24 hours.

Abnormal findings

Low urine magnesium levels may result from malabsorption, acute or chronic diarrhea, diabetic ketoacidosis, dehydration, pancreatitis, advanced renal failure, and primary aldosteronism. They may also result from decreased dietary intake of magnesium.

Elevated urine magnesium levels may result from early chronic renal disease, adrenocortical insufficiency (Addison's disease), chronic alcoholism, or chronic ingestion of magnesium-containing antacids.

Interfering factors

• Contamination of the specimen with toilet tissue or stool
• Magnesium-containing antacids, ethacrynic acid, thiazide diuretics, and aldosterone (possible increase)