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Magnesium (Mg) Test

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 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.
  • Monitor kidney function.
  • 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 who have 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

Don't take medicines 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.
  • Some diuretics.

How It Is Done

A health professional uses a needle to take a blood sample, usually from the arm.

How long the test takes

The test will take a few minutes.

Watch

How It Feels

When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.

Risks

There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.

Results

Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.

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

Credits

Current as of: June 17, 2021

Author: Healthwise Staff
Medical Review:
E. Gregory Thompson MD - Internal Medicine
Adam Husney MD - Family Medicine
Martin J. Gabica MD - Family Medicine