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Mg
Este artigo foi revisto pela última vez em
Este artigo foi modificado pela última vez em 10 de Julho de 2017.
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?
If you have symptoms (such as weakness, irritability, cardiac arrhythmia, nausea, and/or diarrhea) that may be due to too much or too little magnesium or if you have abnormal calcium or potassium levels
Sample Required?
A blood sample drawn from a vein in your arm
What is being tested?
This test measures the amount of magnesium in your blood. Normally, only a very small amount (about 1%) of total body magnesium is present in the blood.

Magnesium is a mineral that is found in every cell of your body. It is vital to energy production, muscle contraction, nerve function, and maintenance of strong bones. About half of the body’s magnesium is combined with calcium and phosphorus to form bone.

A wide variety of foods contain small amounts of magnesium, especially green vegetables such as spinach, and most magnesium in the body comes from dietary sources. The body maintains magnesium levels in its blood, cells, and bone by regulating how much it absorbs from the intestines and by how much it excretes or conserves in the kidneys.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.
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Common Questions
  • How is it used?
    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 (involved with calcium regulation).

    Magnesium levels may be measured frequently to monitor the response to oral or intravenous (IV) magnesium supplements and may be used, 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 if you have symptoms of an abnormally low magnesium level such as muscle weakness, twitching, cramping, confusion, cardiac arrhythmias, and seizures.

    Although dietary deficiencies of magnesium are rare, your doctor may order a magnesium level to check for a deficiency as part of an evaluation of malabsorption, malnutrition, diarrhea, or alcoholism. If you are taking certain medications that can cause the kidneys to excrete magnesium, testing may be performed as well. If magnesium and/or calcium supplementation is necessary, then the magnesium blood level most likely will be checked at intervals to monitor the effectiveness of treatment.

    If you have a kidney disorder or uncontrolled diabetes, your doctor may order magnesium levels to help monitor kidney function and to make sure that you are not excreting or retaining excessive amounts of magnesium.
  • What does the test result mean?
    Low levels of magnesium (hypomagnesemia) in your blood may mean that you are: 1) not getting enough magnesium in your diet; 2) your intestines are not absorbing enough magnesium; or 3) your kidneys are excreting too much magnesium. Deficiencies may be due to:
    • Low dietary intake (seen in the elderly, malnourished, and with alcoholism) 
    • Gastrointestinal disorders (such as Crohn’s disease) 
    • Uncontrolled diabetes 
    • Hypoparathyroidism (underactive parathyroid gland)
    • 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: 

  • Is there anything else I should know?

    Since magnesium is an electrolyte, it may be ordered along with other electrolytes such as sodium, potassium, chloride, bicarbonate (or total CO2), calcium, and phosphorus to evaluate a patient’s electrolyte balance. If magnesium is low, it is not unusual for potassium also to be low.

    Magnesium blood levels may be low normally in the second and third trimesters of pregnancy.

  • Why is my doctor checking my calcium and phosphorus along with my magnesium level?
    Low magnesium levels can affect calcium level regulation. Parathyroid hormone and Vitamin D ordinarily work together along with phosphorus to regulate calcium levels. Low magnesium levels can make low calcium levels more resistant to change.
View Sources
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Thomas, Clayton L., Editor (1997). Taber’s Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

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

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NIH (2001 August 7, Updated). Magnesium [29 paragraphs]. NIH Clinical Center, National Institutes of Health, Facts About Dietary Supplements [On-line information]. Available FTP: http://www.cc.nih.gov/ccc/supplements/magn.html

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Rude, R. [Reviewed] (2001 February 05, Updated). Magnesium [43 paragraphs]. Linus Pauling Institute [On-line information]. Available FTP: http://www.orst.edu/dept/lpi/infocenter/minerals/magnesium/

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Zangwill, M. [Updated] (2001 February 01, Updated). Magnesium in diet [7 paragraphs]. MEDLINEplus Health Information [On-line information]. Available FTP: http://www.nlm.nih.gov/medlineplus/ency/article/002423.htm

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Angelo, S. [Updated] (2001 November 05, Updated). Serum magnesium – test [5 paragraphs]. MEDLINEplus Health Information [On-line information]. Available FTP: http://www.nlm.nih.gov/medlineplus/ency/article/003487.htm

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Merck. Magnesium Metabolism [16 paragraphs]. The Merck Manual of Diagnosis and Therapy [On-line information]. Available FTP: http://www.merck.com/pubs/mmanual/section2/chapter12/12f.htm

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Spengler, R. (2001 June 25, Updated). Magnesium (Mg) [16 paragraphs]. WebMDHealth [On-line information]. Available FTP: http://my.webmd.com/encyclopedia/article/4118.278

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