Magnesium (Mg)

Test Overview

A magnesium test checks the level of magnesium in the blood. Magnesium is an important electrolyte needed for proper muscle, nerve, and enzyme function. It also helps the body make and use energy and is needed to move other electrolytes (potassium and sodium) into and out of cells.

Most of the magnesium in the body is found in the bones and inside the cells. Only a tiny amount of magnesium is normally present in the blood.

Tests for other electrolytes, such as calcium, potassium, sodium, and phosphorus, may be done along with a test for magnesium.

Why It Is Done

A test for magnesium is done to:

  • Find a cause for nerve and muscle problems, such as muscle twitches, irritability, and muscle weakness.
  • Find the cause of symptoms, such as low blood pressure, nausea, vomiting, diarrhea, dizziness, muscle weakness, and slurred speech.
  • Find the cause of heart problems or trouble breathing, especially in people who have kidney disease.
  • Find the cause of a low calcium or potassium level that is not improving with treatment.
  • Look for changes in magnesium levels caused by medicines, such as diuretics.
  • See if people with heart problems need extra magnesium. Low magnesium levels can increase the chances of life-threatening heart rhythm problems.
  • Measure levels when magnesium is being given for medical treatment.

How To Prepare

Many medicines may change the results of this test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take. Do not take medications containing magnesium for at least 3 days before this test. This includes antacids that contain magnesium, laxatives (such as milk of magnesia or Epsom salts), magnesium supplements, and some diuretics.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?) .

How It Is Done

The health professional drawing blood will:

  • Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
  • Clean the needle site with alcohol.
  • Put the needle into the vein. More than one needle stick may be needed.
  • Attach a tube to the needle to fill it with blood.
  • Remove the band from your arm when enough blood is collected.
  • Put a gauze pad or cotton ball over the needle site as the needle is removed.
  • Put pressure on the site and then put on a bandage.

How It Feels

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

Risks

Blood test

There is very little risk of complications from having blood drawn from a vein.

  • You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
  • In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
  • Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.

Results

A magnesium test measures the level of magnesium in the blood. Normal values may vary from lab to lab. Results are usually available in 1 to 2 days.

Normal

Magnesium blood level
Adult:

1.3–2.1 milliequivalents per liter (mEq/L) or 0.65–1.05 millimoles per liter (mmol/L)

Child:

1.4–1.7 mEq/L or 0.7–0.85 mmol/L

Newborn:

1.4–2 mEq/L or 0.7–1.0 mmol/L

Many conditions can change magnesium levels.Your doctor will talk with you about any abnormal results that may be related to your symptoms and medical history.

High values

High magnesium levels may be caused by:

Low values

Low magnesium levels may be caused by:

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • Using medicines that contain magnesium. This includes many antacids, laxatives (such as milk of magnesia or Epsom salts), and magnesium supplements.
  • Medicine. Some antibiotics, insulin, and long-term use of diuretics can lower magnesium levels.
  • Recent intravenous (IV) fluids, such as fluids given during surgery.
  • Pregnancy. Magnesium blood levels may normally be low in the second and third trimesters of pregnancy.

What To Think About

  • A test for magnesium may be done along with testing for other electrolytes, such as calcium, chloride, potassium, and phosphorus. For more information, see the medical tests Calcium (Ca) in Blood, Chloride (Cl), Potassium (K) in Blood, and Phosphate in Blood.
  • The amounts of magnesium and calcium in the body are often closely related.
  • Having low magnesium levels is rare. Symptoms of a magnesium deficiency include weakness, dizziness, irregular heartbeat, shaking, twitching, and seizures. Low magnesium levels are usually caused by not eating enough of the foods that contain magnesium or from problems that block the way food is absorbed from the intestines.

References

Other Works Consulted

  • Chernecky CC, Berger BJ, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed. Philadelphia: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Handbook of Diagnostic Tests (2003). 3rd ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.

Credits

Author Maria G. Essig, MS, ELS
Editor Susan Van Houten, RN, BSN, MBA
Associate Editor Tracy Landauer
Primary Medical Reviewer Kathleen Romito, MD - Family Medicine
Specialist Medical Reviewer Brian D. O'Brien, MD - Internal Medicine
Last Updated April 17, 2008

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