At a Glance
Why Get Tested?
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
A blood sample drawn from a vein in your arm
Test Preparation Needed?
Overnight fasting may be required; follow any instructions that you are given.
The Test Sample
What is being tested?
This test measures the amount of magnesium in the blood. 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.
A wide variety of foods contain small amounts of magnesium, especially green vegetables such as spinach, whole grains, and nuts. The body maintains magnesium levels 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.
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 testing; follow any instructions that you are given.
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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
(Updated 2009 July 13). Magnesium. NIH, Office of Dietary Supplements [On-line information]. Available online at http://ods.od.nih.gov/factsheets/magnesium.asp through http://ods.od.nih.gov. Accessed March 2010.
Vorvick, L. (Updated 2009 March 9). Magnesium in Diet. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/002423.htm. 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 http://www.health.gov/dietaryguidelines/ through http://www.health.gov. Accessed March 2010.
Novello, N. and Blumstein, H. (Updated 2009 August 18). Hypomagnesemia. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/767546-overview through http://emedicine.medscape.com. Accessed March 2010.
Ferry, R. (Updated 2010 January 8). Hypermagnesemia. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/921382-overview through http://emedicine.medscape.com. 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.
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 http://www.cc.nih.gov/ccc/supplements/magn.html through http://www.cc.nih.gov.
Rude, R. [Reviewed] (2001 February 05, Updated). Magnesium. Linus Pauling Institute [On-line information]. Available online at http://www.orst.edu/dept/lpi/infocenter/minerals/magnesium/ through http://www.orst.edu.
Zangwill, M. [Updated] (2001 February 01, Updated). Magnesium in diet. MEDLINEplus Health Information [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/002423.htm.
Angelo, S. [Updated] (2001 November 05, Updated). Serum magnesium test. MEDLINEplus Health Information [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003487.htm.
Merck. Magnesium Metabolism. The Merck Manual of Diagnosis and Therapy [On-line information]. Available online at http://www.merck.com/pubs/mmanual/section2/chapter12/12f.htm through http://www.merck.com.
Spengler, R. (2001 June 25, Updated). Magnesium (Mg). WebMDHealth [On-line information]. Available online at http://my.webmd.com/encyclopedia/article/4118.278 through http://my.webmd.com.