Magnesium (Mg) excretion controls serum magnesium balance. Magnesium also helps regulate calcium absorption and bone and teeth integrity. Urinary magnesium excretion is diet dependent.

This test evaluates magnesium metabolism, investigates electrolyte status, and is a component of a workup for nephrolithiasis. It is useful for assessing the cause of an abnormal serum magnesium level. The magnesium load test is used to identify magnesium deficiency in individuals with normal kidney function.

75–150 mg/24 hr or 3.00–6.00 mmol/d

Values are diet dependent.

1. Collect a 24-hour urine specimen in a metal-free and acid-rinsed container. The pH must be <2.

2. Record exact starting and ending times.

3. See Long-Term, Timed Urine Specimen (2-Hour, 24-Hour) for 24-hour urine collection guidelines.

4. For the magnesium load test, the patient is given 30 mmol of MgSO₄ in 1.0 L of normal saline via IV over an 8-hour period. Urine is collected for 24 hours beginning with the start of the IV.

1. Increased urine magnesium is associated with:

   1. Increased blood alcohol

   2. Bartter syndrome

   3. Chronic glomerulonephritis

2. Decreased urine magnesium is associated with:

   1. Malabsorption

   2. Long-term chronic alcoholism

   3. Long-term parenteral therapy

   4. Magnesium deficiency

   5. CKD

   6. Hypoparathyroidism

   7. Hypercalciuria

   8. Decreased kidney function (e.g., Addison disease)

Pretest Patient Care

1. Explain purpose of test, procedure for urine collection, and interfering factors.

2. Instruct that the specimen will be unacceptable if it comes in contact with any type of metal.

3. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

1. Increased magnesium levels are associated with:

   1. Corticosteroids

   2. Cisplatin therapy

   3. Thiazide diuretics

   4. Amphotericin (see Appendix E)

   5. Blood in urine

2. Decreased magnesium levels: Many drugs affect test outcomes (see Appendix E).