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Also known as: Mg; Mag
Formal name: Magnesium

At a Glance

Why Get Tested?

To evaluate the level of magnesium in your blood and to help determine the cause of abnormal levels of magnesium, calcium and/or potassium

When to Get Tested?

When you have symptoms such as weakness, irritability, cardiac arrhythmia, nausea, and/or diarrhea that may be due to too much or too little magnesium; when you have abnormal calcium or potassium levels

Sample Required?

A blood sample drawn from a vein in your arm; sometimes a timed urine collection that most often requires a 24-hour sample

Test Preparation Needed?

For a blood sample, overnight fasting may be required; follow any instructions that you are given.

The Test Sample

What is being tested?

Magnesium is a mineral that is vital for energy production, muscle contraction, nerve function, and the maintenance of strong bones. It comes into the body through the diet and is absorbed by the small intestine and colon. Magnesium is stored in the bones, cells, and tissues. Normally, only about 1% of total body magnesium is present in the blood and this makes it difficult to get an accurate measurement of total magnesium content from blood tests alone. However, this test is still useful for evaluating a person's magnesium status.

A wide variety of foods contain small amounts of magnesium, especially green vegetables such as spinach, as well as whole grains and nuts. Foods that have dietary fiber are usually also sources of magnesium. The body maintains its magnesium level by regulating how much it absorbs and how much it excretes or conserves in the kidneys.

Magnesium deficiencies (hypomagnesemia) may be seen with malnutrition, conditions that cause malabsorption, and with excess loss of magnesium by the kidneys. Magnesium excess (hypermagnesemia) may be seen with the ingestion of antacids that contain magnesium and with decreased ability of the kidneys to excrete magnesium.

Someone with mild to moderate magnesium deficiency may have no or few nonspecific symptoms. Persistent or severe deficiencies can cause nausea, loss of appetite, fatigue, confusion, muscle cramps, seizures, changes in heart rate, and numbness or tingling. They can also affect calcium metabolism and exacerbate calcium deficiencies. Symptoms of excess magnesium can be similar to those of deficiency and include nausea, muscle weakness, loss of appetite, and an irregular heart rate.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm. Sometimes a timed urine sample is obtained, such as a 24-hour collection.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

Overnight fasting may be required prior to having a blood sample drawn; follow any instructions that you are given.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Magnesium, 24 Hour, Urine. Mayo Clinic. Available online at through Accessed October 2013.

Elin, R.J. Assessment of magnesium status for diagnosis and therapy. National Center for Biotechnology Information PubMed Database. Available online at through Accessed October 2013.

Dietary Supplement Fact Sheet: Magnesium. National Institutes of Health, Office of Dietary Supplements. Available online at through Accessed October 2013.

Serum magnesium. MedlinePlus Medical Encyclopedia. Available online at Accessed December 2013.

Magnesium measurement, urine. MUSC Health. Available online at through Accessed December 2013.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

NIH (2001 August 7, Updated). Magnesium. NIH Clinical Center, National Institutes of Health, Facts About Dietary Supplements [On-line information]. Available online at through

Rude, R. [Reviewed] (2001 February 05, Updated). Magnesium. Linus Pauling Institute [On-line information]. Available online at through

Zangwill, M. [Updated] (2001 February 01, Updated). Magnesium in diet. MEDLINEplus Health Information [On-line information]. Available online at

Angelo, S. [Updated] (2001 November 05, Updated). Serum magnesium test. MEDLINEplus Health Information [On-line information]. Available online at

Merck. Magnesium Metabolism. The Merck Manual of Diagnosis and Therapy [On-line information]. Available online at through

Spengler, R. (2001 June 25, Updated). Magnesium (Mg). WebMDHealth [On-line information]. Available online at through

(Updated 2009 July 13). Magnesium. NIH, Office of Dietary Supplements [On-line information]. Available online at through Accessed March 2010.

Vorvick, L. (Updated 2009 March 9). Magnesium in Diet. MedlinePlus Medical Encyclopedia [On-line information]. Available online at Accessed March 2010.

(2005 January). U.S. Department of Health and Human Services and U.S. Department of Agriculture. Dietary Guidelines for Americans, 2005. 6th Edition, Washington, DC: U.S. Government Printing Office. Chapter 2 Adequate Nutrients Within Calorie Needs [On-line information]. Available online at through Accessed March 2010.

Novello, N. and Blumstein, H. (Updated 2009 August 18). Hypomagnesemia. eMedicine [On-line information]. Available online at through Accessed March 2010.

Ferry, R. (Updated 2010 January 8). Hypermagnesemia. eMedicine [On-line information]. Available online at through Accessed March 2010.

Pagana, K. D. & Pagana, T. J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 640-641.

Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 706-709.