At a Glance
Why Get Tested?
To evaluate the level of magnesium in your blood and to help determine the cause of abnormal calcium and/or potassium levels
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
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.
The Test
How is it used?
A magnesium test is used to measure the level of magnesium in the blood. Abnormal levels of magnesium are most frequently seen in conditions or diseases that cause impaired or excessive excretion of magnesium by the kidneys or that cause impaired absorption in the intestines. Magnesium levels may be checked as part of an evaluation of the severity of kidney problems and/or of uncontrolled diabetes and may help in the diagnosis of gastrointestinal disorders.
Since a low magnesium level can, over time, cause persistently low calcium and potassium levels, it may be checked to help diagnose problems with calcium, potassium, phosphorus, and/or parathyroid hormone – another component of calcium regulation.
Magnesium levels may be measured frequently to monitor the response to oral or intravenous (IV) magnesium supplements and may be ordered, along with calcium and phosphorus testing, to monitor calcium supplementation.
When is it ordered?
Magnesium testing may be ordered as a follow up to chronically low levels of calcium and potassium. It also may be ordered when a person has symptoms that may be due to a magnesium deficiency, such as muscle weakness, twitching, cramping, confusion, cardiac arrhythmias, and seizures.
A doctor may order a magnesium level to check for a deficiency as part of an evaluation of malabsorption, malnutrition, diarrhea, or alcoholism. When someone is taking medications that can cause the kidneys to excrete magnesium, testing may be performed as well. When magnesium and/or calcium supplementation is necessary, then the magnesium blood level may be checked at intervals to monitor the effectiveness of treatment.
When someone has a kidney disorder or uncontrolled diabetes, magnesium levels may be ordered periodically, along with kidney function tests such as a BUN and creatinine, to help monitor kidney function and to make sure that the person is not excreting or retaining excessive amounts of magnesium.
What does the test result mean?
Low levels of magnesium may indicate that a person is not consuming or absorbing enough or is excreting too much magnesium. Deficiencies are typically seen with:
- Low dietary intake (seen in the elderly, malnourished, and with alcoholism)
- Gastrointestinal disorders (such as Crohn's disease)
- Uncontrolled diabetes
- Hypoparathyroidism
- Long-term diuretic use
- Prolonged diarrhea
- Post surgery
- Severe burns
- Toxemia of pregnancy
Increased levels of magnesium are rarely due to dietary sources but are usually the result of an excretion problem or excessive supplementation. Increased levels are seen in:
- Kidney failure
- Hyperparathyroidism
- Hypothyroidism
- Dehydration
- Diabetic acidosis (when first seen)
- Addison's disease
- Use of magnesium-containing antacids or laxatives
Is there anything else I should know?
Since magnesium is an electrolyte, a magnesium test may be ordered along with other electrolytes such as sodium, potassium, chloride, bicarbonate (or total CO2), calcium, and phosphorus to evaluate a person's electrolyte balance. If magnesium is low, it is not unusual for potassium also to be low.
Magnesium blood levels tend to be decreased in the second and third trimesters of pregnancy.
Normal levels of magnesium do not necessarily reflect body stores of magnesium. The body attempts to keep blood magnesium levels relatively stable and will release magnesium from bone and tissues to accomplish this. So blood levels may be normal with early magnesium deficiencies.
Drugs that can increase magnesium levels include lithium, aspirin, thyroid medication, some antibiotics, and products that contain magnesium. Drugs that can decrease magnesium levels include digoxin, cyclosporine, diuretics, insulin, some antibiotics, laxatives, and phenytoin.
Common Questions
Ask a Laboratory Scientist
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Article Sources
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.
