Patient Test Information


Also known as:

Mg; Mag

Formal name:


Related tests:

Calcium, Potassium, Phosphorus, PTH, Vitamin D

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.

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 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 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.

A health 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 someone 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 excreting too much. 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 blood 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 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 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.

  1. Is everyone tested for magnesium?

    Magnesium is not monitored as frequently as some other minerals. It tends to be ordered primarily when a person has an abnormal calcium level and when a person has symptoms that may be due to a magnesium deficiency or excess.

  2. Should I take magnesium supplements? And if so, how much?

    People will absorb, use, and excrete 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.