Magnesium, Urine

Synonym/acronym: Urine Mg²⁺.

Common use

To assess magnesium levels related to renal function.

Specimen

Urine (5 mL) from a random or timed specimen collected in a clean plastic collection container with 6N hydrochloride as a preservative.

Normal findings

(Method: Spectrophotometry)

Conventional Units	SI Units (Conventional Units × 0.4114)
20–200 mg/24 hr	8.2–82.3 mmol/24 hr

Description

Magnesium is required as a cofactor in numerous crucial enzymatic processes, such as protein synthesis, nucleic acid synthesis, and muscle contraction. Magnesium is also required for the use of adenosine diphosphate as a source of energy. It is the fourth most abundant cation and the second most abundant intracellular ion. Magnesium is needed for the transmission of nerve impulses and muscle relaxation. It controls absorption of sodium, potassium, calcium, and phosphorus; utilization of carbohydrate, lipid, and protein; and activation of enzyme systems that enable the B vitamins to function. Magnesium is also essential for oxidative phosphorylation, nucleic acid synthesis, and blood clotting. Urine magnesium levels reflect magnesium deficiency before serum levels. Magnesium deficiency severe enough to cause hypocalcemia and cardiac arrhythmias can exist despite normal serum magnesium levels.

Regulating electrolyte balance is one of the major functions of the kidneys. In normally functioning kidneys, urine levels increase when serum levels are high and decrease when serum levels are low to maintain homeostasis. Analyzing these urinary levels can provide important clues as to the functioning of the kidneys and other major organs. Tests for electrolytes, such as magnesium, in urine usually involve timed urine collections over a 12- or 24-hr period. Measurement of random specimens may also be requested.

This procedure is contraindicated for

N/A

Indications

Determine the potential cause of renal calculi
Evaluate known or suspected endocrine disorder
Evaluate known or suspected renal disease
Evaluate magnesium imbalance
Evaluate a malabsorption problem

Potential diagnosis

Increased in

Alcoholism (related to impaired absorption and increased urinary excretion)
Bartter’s syndrome (inherited defect in renal tubules that results in urinary wasting of potassium and magnesium)
Transplant recipients on cyclosporine and prednisone (related to increased excretion by the kidney)
Use of corticosteroids (related to increased excretion by the kidney)
Use of diuretics (related to increased urinary excretion)

Decreased in

(related to diminished ability of renal tubules to reabsorb magnesium)

(related to inadequate intestinal absorption)

(related to diminished renal absorption)

Critical findings

N/A

Interfering factors

Drugs that may increase urine magnesium levels include cisplatin, cyclosporine, ethacrynic acid, furosemide, mercaptomerin, mercurial diuretics, thiazides, torsemide, and triamterene.
Drugs that may decrease urine magnesium levels include amiloride, angiotensin, oral contraceptives, parathyroid extract, and phosphates.
Magnesium levels follow a circadian rhythm, and for this reason 24-hr collections are recommended.
All urine voided for the timed collection period must be included in the collection, or else falsely decreased values may be obtained. Compare output records with volume collected to verify that all voids were included in the collection.

Nursing Implications and Procedure

Pretest

Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
Patient Teaching: Inform the patient this test can assist in evaluating magnesium balance.
Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
Obtain a history of the patient’s endocrine, gastrointestinal, and genitourinary systems; symptoms; and results of previously performed laboratory tests and diagnostic and surgical procedures.
Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values).
Review the procedure with the patient. Provide a nonmetallic urinal, bedpan, or toilet-mounted collection device. Address concerns about pain related to the procedure. Explain to the patient that there should be no discomfort during the procedure.
Usually a 24-hr time frame for urine collection is ordered. Inform the patient that all urine must be saved during that 24-hr period. Instruct the patient not to void directly into the laboratory collection container. Instruct the patient to avoid defecating in the collection device and to keep toilet tissue out of the collection device to prevent contamination of the specimen. Place a sign in the bathroom to remind the patient to save all urine.
Instruct the patient to void all urine into the collection device and then to pour the urine into the laboratory collection container. Alternatively, the specimen can be left in the collection device for a health-care staff member to add to the laboratory collection container.
Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
Instruct the patient to avoid excessive exercise and stress during the 24-hr collection of urine.
Note that there are no food, fluid, or medication restrictions unless by medical direction.

Intratest

Potential complications: N/A
Ensure that the patient has complied with activity restrictions during the procedure.
Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
Instruct the patient to cooperate fully and to follow directions.
Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection.

Random Specimen (Collect in Early Morning)

Clean-Catch Specimen
Instruct the male patient to (1) thoroughly wash his hands, (2) cleanse the meatus, (3) void a small amount into the toilet, and (4) void directly into the specimen container.
Instruct the female patient to (1) thoroughly wash her hands; (2) cleanse the labia from front to back; (3) while keeping the labia separated, void a small amount into the toilet; and (4) without interrupting the urine stream, void directly into the specimen container.
Indwelling Catheter
Put on gloves. Empty drainage tube of urine. It may be necessary to clamp off the catheter for 15 to 30 min before specimen collection. Cleanse specimen port with antiseptic swab, and then aspirate 5 mL of urine with a 21- to 25-gauge needle and syringe. Transfer urine to a sterile container.
Timed Specimen
Obtain a clean 3-L urine specimen container, toilet-mounted collection device, and plastic bag (for transport of the specimen container). The specimen must be refrigerated or kept on ice throughout the entire collection period. If an indwelling urinary catheter is in place, the drainage bag must be kept on ice.
Begin the test between 6 and 8 a.m. if possible. Collect first voiding and discard. Record the time the specimen was discarded as the beginning of the timed collection period. The next morning, ask the patient to void at the same time the collection was started and add this last voiding to the container. Urinary output should be recorded throughout the collection time.
If an indwelling catheter is in place, replace the tubing and container system at the start of the collection time. Keep the container system on ice during the collection period, or empty the urine into a larger container periodically during the collection period; monitor to ensure continued drainage, and conclude the test the next morning at the same hour the collection was begun.
At the conclusion of the test, compare the quantity of urine with the urinary output record for the collection; if the specimen contains less than what was recorded as output, some urine may have been discarded, invalidating the test.
Include on the collection container’s label the amount of urine, test start and stop times, and ingestion of any foods or medications that can affect test results.
Promptly transport the specimen to the laboratory for processing and analysis.

Post-Test

Inform the patient that a report of the results will be made available to the requesting health-care provider (HCP), who will discuss the results with the patient.
Nutritional Considerations: Educate the magnesium-deficient patient regarding good dietary sources of magnesium, such as green vegetables, seeds, legumes, shrimp, and some bran cereals. Advise the patient that high intake of substances such as phosphorus, calcium, fat, and protein interferes with the absorption of magnesium.
Instruct the patient to report any signs or symptoms of electrolyte imbalance, such as dehydration, diarrhea, vomiting, or prolonged anorexia.
Recognize anxiety related to test results. Discuss the implications of abnormal test results on the patient’s lifestyle. Provide teaching and information regarding the clinical implications of the test results, as appropriate.
Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family.
Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

Related tests include ACTH, aldosterone, angiography renal, anion gap, BUN, calcium, calculus kidney stone panel, CT renal, cortisol, creatinine, glucose, IVP, magnesium, osmolality, PTH, phosphorus, potassium, renin, renogram, sodium, troponin, UA, US kidney, and vitamin D.
Refer to the Endocrine, Gastrointestinal, and Genitourinary systems tables at the end of the book for related tests by body system.