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At a Glance
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; when magnesium is given for medical treatment
A blood sample drawn from a vein in your arm; sometimes a timed urine collection that most often requires a 24-hour sample collection
Test Preparation Needed?
For a blood sample, overnight fasting may be required; follow any instructions that you are given.
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 it used?
A magnesium test is used to measure the level of magnesium in the blood (or sometimes urine). 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 blood 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. The test for magnesium 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 blood 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. Magnesium depletion is known to be linked to severe alcohol intake or due to irregular or bad absorption of magnesium in the intestine.
A healthcare practitioner 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, the level of magnesium in the blood may be checked at intervals to monitor the effectiveness of treatment.
When an individual has a kidney disorder or uncontrolled diabetes, a magnesium test 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 blood levels of magnesium may indicate that a person is not consuming or absorbing enough magnesium or is eliminating too much from the body.
Decreased magnesium levels are typically seen with:
- Low dietary intake, which may be seen in the elderly, people who are malnourished or those with alcoholism
- Digestive disorders (such as Crohn disease)
- Uncontrolled diabetes
- Long-term diuretic use
- Prolonged diarrhea
- Severe burns
High blood levels of magnesium are rarely due to dietary sources but are usually the result of excessive supplementation or a problem with eliminating it from the body.
Increased magnesium levels are seen in:
- End-stage renal disease (ESRD)
- Diabetic acidosis (when first seen)
- Addison 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 total 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, the blood level 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.
What are symptoms of low magnesium?
Low magnesium may cause signs and symptoms such as muscle weakness, twitching, cramping, confusion, changes in heart rate (cardiac arrhythmias), and seizures.
Should I take magnesium supplements? And if so, how much?
People will absorb and use magnesium at different rates and may be taking drugs that affect magnesium levels. You should talk to your health care provider about what is appropriate for your situation.
Sources Used in Current Review
2017 review performed by Jagadish K. Boppisetti, PhD, Senior Scientist.
(Updated: February 11, 2016) Magnesium Fact Sheet for Health Professionals. Office of Dietary Supplements (ODS), National Institutes of Health. Available online at https://ods.od.nih.gov/factsheets/Magnesium-HealthProfessional/. Accessed June 2017.
Rude RK. Magnesium. In: Ross AC, Caballero B, Cousins RJ, Tucker KL, Ziegler TR, eds. Modern Nutrition in Health and Disease. 11th ed. Baltimore, Mass: Lippincott Williams & Wilkins; 2012:159-175.
Guerra M, et al. Therapeutic Uses of Magnesium. Am Fam Physician. 2009 Jul 15;80(2):157-162. Available online at http://www.aafp.org/afp/2009/0715/p157.html. Accessed on 06/29/2017.
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