Magnesium, Blood

Magnesium, Blood

Synonym/acronym: Mg2+.

Common use

To assess electrolyte balance related to magnesium levels to assist in diagnosis, monitoring diseases, and therapeutic interventions such as hemodialysis.


Serum (1 mL) collected in a gold-, red-, or red/gray-top tube.

Normal findings

(Method: Spectrophotometry)
AgeConventional UnitsSI Units (Conventional Units × 0.4114)
Newborn1.7–2.5 mg/dL0.7–1 mmol/L
Child1.7–2.3 mg/dL0.7–0.95 mmol/L
Adult1.6–2.6 mg/dL0.66–1.07 mmol/L


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.

This procedure is contraindicated for



  • Determine electrolyte balance in renal failure and chronic alcoholism
  • Evaluate cardiac arrhythmias (decreased magnesium levels can lead to excessive ventricular irritability)
  • Evaluate known or suspected disorders associated with altered magnesium levels
  • Monitor the effects of various drugs on magnesium levels

Potential diagnosis

Increased in

  • Addison’s disease (related to insufficient production of aldosterone; decreased renal excretion)
  • Adrenocortical insufficiency (related to decreased renal excretion)
  • Dehydration (related to hemoconcentration)
  • Diabetic acidosis (severe) (related to acid-base imbalance)
  • Hypothyroidism (pathophysiology is unclear)
  • Massive hemolysis (related to release of intracellular magnesium; intracellular concentration is three times higher than normal plasma levels)
  • Overuse of antacids (related to excessive intake of magnesium-containing antacids)
  • Renal insufficiency (related to decreased urinary excretion)
  • Tissue trauma

Decreased in

    Alcoholism (related to increased renal excretion and possible insufficient dietary intake) Diabetic acidosis (insulin treatment lowers blood glucose and appears to increase intracellular transport of magnesium) Glomerulonephritis (chronic) (related to diminished renal function; magnesium is reabsorbed in the renal tubules) Hemodialysis (related to loss of magnesium due to dialysis treatment) Hyperaldosteronism (related to increased excretion) Hypocalcemia (decreased magnesium is associated with decreased calcium and vitamin D levels) Hypoparathyroidism (related to decreased calcium) Inadequate intake Inappropriate secretion of antidiuretic hormone (related to fluid overload) Long-term hyperalimentation Malabsorption (related to impaired absorption of calcium and vitamin D) Pancreatitis (secondary to alcoholism) Pregnancy Severe loss of body fluids (diarrhea, lactation, sweating, laxative abuse)

Critical findings


  • Less than 1.2 mg/dL (SI: Less than 0.5 mmol/L)
  • Greater than 4.9 mg/dL (SI: Greater than 2 mmol/L)
  • Children

  • Less than 1.2 mg/dL (SI: Less than 0.5 mmol/L)
  • Greater than 4.3 mg/dL (SI: Greater than 1.8 mmol/L)
  • Note and immediately report to the health-care provider (HCP) any critically increased or decreased values and related symptoms.

  • Symptoms such as tetany, weakness, dizziness, tremors, hyperactivity, nausea, vomiting, and convulsions occur at decreased (less than 1.2 mg/dL) concentrations. Electrocardiographic (ECG) changes (prolonged P-R and Q-T intervals; broad, flat T waves; and ventricular tachycardia) may also occur. Treatment may include IV or oral administration of magnesium salts, monitoring for respiratory depression and areflexia (IV administration of magnesium salts), and monitoring for diarrhea and metabolic alkalosis (oral administration to replace magnesium).

  • Respiratory paralysis, decreased reflexes, and cardiac arrest occur at grossly elevated (greater than 15 mg/dL) levels. ECG changes, such as prolonged P-R and Q-T intervals, and bradycardia may be seen. Toxic levels of magnesium may be reversed with the administration of calcium, dialysis treatments, and removal of the source of excessive intake.

Interfering factors

  • Drugs that may increase magnesium levels include acetylsalicylic acid and progesterone.
  • Drugs that may decrease magnesium levels include albuterol, aminoglycosides, amphotericin B, bendroflumethiazide, chlorthalidone, cisplatin, citrates, cyclosporine, digoxin, gentamicin, glucagon, and oral contraceptives.
  • Magnesium is present in higher intracellular concentrations; therefore, hemolysis will result in a false elevation in values and such specimens should be rejected for analysis.
  • Specimens should never be collected above an IV line because of the potential for dilution when the specimen and the IV solution combine in the collection container, falsely decreasing the result. There is also the potential of contaminating the sample with the substance of interest, if it is present in the IV solution, falsely increasing the result.

Nursing Implications and Procedure

Potential nursing problems

ProblemSigns & SymptomsInterventions
Fluid volume (Related to metabolic imbalances associated with disease process)Deficiency: decreased urinary output, fatigue, and sunken eyes, dark urine, decreased blood pressure, increased heart rate, and altered mental status. Overload: Edema, shortness of breath, increased weight, ascites, rales, rhonchi, and diluted laboratory valuesRecord daily weight and monitor trends; record accurate intake and output; collaborate with physician with administration of IV fluids to support hydration; monitor laboratory values that reflect alterations in fluid status (potassium, blood urea nitrogen, creatinine, calcium, hemoglobin, and hematocrit); manage underlying cause of fluid alteration; monitor urine characteristics and respiratory status; establish baseline assessment data; collaborate with physician to adjust oral and IV fluids to provide optimal hydration status; administer replacement electrolytes as ordered; monitor serum magnesium levels
Nutrition (Related to excess caloric intake with large amounts of dietary sodium and fat; cultural lifestyle; overeating associated with anxiety, depression, compulsive disorder; genetics; inadequate or unhealthy food resources)Observable obesity; high fat or sodium food selections; high BMI; high consumption of ethnic foods; sedentary lifestyle; dietary religious beliefs and food selections; binge eating; diet high in refined sugar; repetitive dieting and failureDiscuss ideal body weight and the purpose and relationship between ideal weight and caloric intake to support cardiac health; review ways to decrease intake of saturated fats and increase intake of polyunsaturated fats; discuss limiting cholesterol intake to less than 300 mg per day; discuss limiting the intake of refined processed sugar; teach limiting sodium intake to the health-care provider's recommended restriction; encourage intake of fresh fruits and vegetables, unprocessed carbohydrates, poultry, and grains
Electrolyte imbalance (Related to metabolic imbalance)Excess: Nausea; vomiting; diarrhea; diaphoresis; flushing; sensation of heat; decreased mental functioning; weakness; drowsiness; hypotension; bradycardia; respiratory depression; coma. Deficit: nystagmus; fatigue; convulsions; weakness; numbnessCorrelate magnesium imbalance with disease process, nutritional intake, renal function, medications; monitor ECG status; monitor for respiratory changes; monitor for GI changes; minimize metabolic complications; provide a safe environment to prevent injury; collaborate with the pharmacist and health-care provider for appropriate pharmacologic interventions; adjust medication dosage to compensate for renal impairment; collaborate with dietician for dietary modifications
Cardiac output (Related to increased preload; increased afterload; impaired cardiac contractility; cardiac muscle disease; altered cardiac conduction)Decreased peripheral pulses; decreased urinary output; cool clammy skin; tachypnea; dyspnea; edema; altered level of consciousness; abnormal heart sounds; crackles in lungs; decreased activity tolerance; weight gain; fatigue; hypoxiaAssess peripheral pulses and capillary refill; monitor blood pressure and check for orthostatic changes; assess respiratory rate, breath sounds, and orthopnea; assess skin color and temperature; assess level of consciousness; monitor urinary output; use pulse oximetry to monitor oxygenation; monitor sodium and potassium levels; monitor BNP levels; administer ordered angiontensin-converting enzyme (ACE) inhibitors, beta-blockers, diuretics, aldosterone antagonists, and vasodilators; provide oxygen administration


  • 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 the evaluation of electrolyte 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 cardiovascular, endocrine, gastrointestinal, genitourinary, and reproductive 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 online at DavisPlus).
  • Review the procedure with the patient. Inform the patient that specimen collection takes approximately 5 to 10 min. Address concerns about pain and explain that there may be some discomfort during the venipuncture.
  • Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • Note that there are no food, fluid, or medication restrictions unless by medical direction.


  • Potential complications: N/A
  • 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. Direct the patient to breathe normally and to avoid unnecessary movement.
  • 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. Perform a venipuncture.
  • Remove the needle and apply direct pressure with dry gauze to stop bleeding. Observe/assess venipuncture site for bleeding or hematoma formation and secure gauze with adhesive bandage.
  • Promptly transport the specimen to the laboratory for processing and analysis.


  • Inform the patient that a report of the results will be made available to the requesting 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.
  • 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.
  • Patient Education

    • Instruct the patient to report any signs or symptoms of electrolyte imbalance, such as dehydration, diarrhea, vomiting, or prolonged anorexia.
    • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP.
    • Recognize anxiety related to test results and answer any questions or address any concerns voiced by the patient or family.
    • Educate the patient regarding access to nutritional counseling services.
    • Provide contact information, if desired, for the Institute of Medicine of the National Academies (
    • Teach the importance of maintaining an appropriate magnesium level to their overall health.
    • Teach the patient that renal disease can contribute to an elevated magnesium level.
  • Expected Patient Outcomes

    • Knowledge
    • States understanding that dietary magnesium intake may need to be decreased to prevent hypomagnesaemia
    • States understanding that renal disease can contribute to high serum magnesium levels
    • Skills
    • Accurately selects a diet that is appropriate to support their personal magnesium needs
    • Identifies foods in their diet that should be added or deleted to support an appropriate magnesium level
    • Attitude
    • Complies with the recommendation to take dietary supplements to improved magnesium levels
    • Discusses the possibility of hemodialysis as needed to manage serum magnesium levels

Related Monographs

  • Related tests include ACTH, aldosterone, anion gap, antiarrhythmic drugs, AST, BUN, calcium, calculus kidney stone panel, CBC WBC count and differential, cortisol, CRP, CK and isoenzymes, creatinine, glucose, homocysteine, LDH and isoenzymes, magnesium urine, myoglobin, osmolality, PTH, phosphorus, potassium, renin, sodium, troponin, US abdomen, and vitamin D.
  • Refer to the Cardiovascular, Endocrine, Gastrointestinal, Genitourinary, and Reproductive systems tables at the end of the book for related tests by body system.
Handbook of Laboratory and Diagnostic Tests, © 2013 Farlex and Partners
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