Test Quick Guide

Magnesium is a mineral vital for energy production, muscle contraction, nerve function, and the maintenance of strong bones. It enters the body through the diet and is absorbed by the small intestine and colon. A magnesium blood test can be used to evaluate the mineral level in your blood and help determine the cause of abnormal levels of magnesium, calcium, and/or potassium.

About the Test

Purpose of the test

A test measures magnesium level in the blood (or sometimes urine). Abnormal magnesium levels are most frequently seen in conditions or diseases that cause impaired or excessive excretion of the mineral by the kidneys, or impaired absorption in the intestines.

Magnesium levels may be checked as part of an evaluation of the severity of kidney problems and/or uncontrolled diabetes and may help diagnose 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 magnesium test may be ordered, along with calcium and phosphorus testing, to monitor calcium supplementation.

What does the test measure?

Magnesium is stored in the bones, cells, and tissues. Normally, only about 1% of total body magnesium is present in the liquid portion of blood, making it difficult to accurately measure total magnesium content from blood tests alone. However, this test is still useful for evaluating your magnesium status.

A variety of foods contain small amounts of magnesium, especially green vegetables such as spinach, 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, excretes, or conserves in the kidneys.

Magnesium deficiencies (hypomagnesemia) may be seen with malnutrition, conditions that cause malabsorption, and excess mineral loss by the kidneys. An excess of magnesium (hypermagnesemia) may happen with the ingestion of antacids that contain magnesium and with a decreased ability of the kidneys to excrete the mineral. But clinically significant hypermagnesemia is rare due to the kidney’s ability to increase excretion when needed.

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.

When should I get this test?

Magnesium testing may be a follow-up to chronically low blood calcium and potassium levels. It also may be ordered when you have symptoms possibly 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 irregular or bad absorption of magnesium in the intestine.

A health care practitioner may order a magnesium test to check for a deficiency as part of an evaluation of malabsorption, malnutrition, diarrhea, or alcoholism. Testing may also be performed when someone is taking medications that can cause the kidneys to excrete magnesium.

If 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 you have a kidney disorder or uncontrolled diabetes, a magnesium test may be ordered periodically. Tests such as a BUN and creatinine may also be used to help monitor kidney function and ensure you are not excreting or retaining excessive amounts of magnesium.

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 your electrolyte balance. If magnesium is low, it is not unusual for potassium also to be low, because prolonged hypomagnesemia (low magnesium in blood) can cause refractory hypokalemia (low potassium in blood).

Magnesium blood levels decrease in the second and third trimesters of pregnancy due to dilution of blood.

Normal magnesium levels do not necessarily reflect the total body stores of the mineral. The body attempts to keep blood magnesium levels relatively stable and will release it 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, laxatives, and magnesium salts. Digoxin, cyclosporine, diuretics, insulin, some antibiotics, phenytoin, antacids, and other drugs can decrease magnesium levels.

Finding a Magnesium Test

How can I get a magnesium test?

The standard magnesium test uses a blood sample obtained with a blood draw at your doctor’s office, a hospital, or a laboratory. Magnesium may be tested on its own or as part of a broader health check or heavy metal panel.

Magnesium red blood cell (RBC) testing can also measure the magnesium level in the red blood cells. Such tests can better assess deficiency than the magnesium blood test.

Testing is normally done after being prescribed by a doctor or other health professional. You can also order a magnesium blood test or magnesium RBC test online.

Can I take the test at home?

While you order a magnesium test online, you will need to visit a local laboratory to have your blood sample taken. LabCorp is a leading provider of in-person magnesium tests.

How much does the test cost?

A magnesium test’s cost varies depending on the type of test and the lab you use to collect your sample. The final cost may include several components, including office visits, technician fees for taking your sample, and/or charges for laboratory evaluation.

Your insurance company may pay for some or all of these costs if your doctor prescribes your magnesium test. For the most definitive information about likely costs, talk with your doctor’s office and medical insurance company.

Taking a Magnesium Test

For a magnesium test, a blood sample is taken in a medical lab, hospital, or doctor’s office.

Before the test

If you are having a blood test for magnesium alone, you usually will not have to follow any special preparation for the test. However, for magnesium RBC tests, you may need to refrain from taking vitamins or mineral herbal supplements for at least one week before sample collection.

Similarly, if you are having magnesium tested as part of a panel test like a basic or comprehensive metabolic panel, you may need to fast for eight to 12 hours beforehand. Check with your doctor about whether you can eat and drink before the blood draw.

During the test

When a blood sample is taken from a vein in your arm, an elastic band will usually be tied around your upper arm to increase blood flow and make it easier to access the vein. The technician will wipe your skin near the vein with an antiseptic, then insert a needle. After the vial of blood is filled, the needle will be taken out.

The total blood draw usually lasts only a few minutes. There may be some pain during the procedure, but most people only feel a brief sting when the needle is inserted.

After the test

You can return to most normal activities after the test is completed. If you feel slight pain or notice bruising around the puncture site, this normally goes away quickly.

Magnesium Test Results

Receiving test results

In most cases, magnesium test results are available within a few business days. Results can be sent by mail or made accessible through online health portals. You may also receive a call or email from your doctor to either review your results or schedule a follow-up appointment.

Interpreting test results

Low blood levels of magnesium may indicate that you are not consuming or absorbing enough of the mineral or are eliminating too much from the body.

Decreased magnesium levels are typically seen with:

  • Low dietary intake, which may be seen in the elderly, if you are malnourished or have chronic alcoholism
  • Digestive disorders (such as Crohn’s disease, enteric fistula)
  • Uncontrolled diabetes
  • Hypoparathyroidism (eg, post-parathyroid surgery)
  • Long-term diuretic use
  • Prolonged diarrhea
  • Chronic glomerulonephritis
  • Post-surgery
  • Severe burns
  • Pre-eclampsia

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)
  • Hyperparathyroidism
  • Hypothyroidism
  • Tissue trauma
  • Dehydration
  • Diabetic acidosis (when first seen)
  • Addison’s disease
  • Use of magnesium-containing antacids or laxatives
  • Prolonged use of aspirin

Asking questions can help you understand the meaning of your magnesium test results. When you talk with your doctor, some of these questions may be helpful to review:

  • Was my magnesium level low, normal, or high?
  • Were any other measurements taken along with magnesium? If so, were they normal or abnormal?
  • If my magnesium was too high or too low, what is the most likely cause?
  • Are there any follow-up tests that you recommend?
  • Should I have another magnesium test?
  • Should I make any changes to my diet?


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