Magnesium Blood Test

Why this test may be ordered

• To check magnesium level if your health care provider thinks your levels may not be normal
• To help find the cause of abnormal levels of other minerals such as calcium, potassium, and phosphorus
• To monitor health if you have a condition linked to abnormal magnesium levels (e.g., chronic kidney disease, diabetes, alcohol use disorder, malabsorption syndrome, malnutrition, chronic diarrhea, low potassium/calcium)
• To monitor if you take medicines that can decrease magnesium (diuretics, certain antibiotics, proton pump inhibitors) or increase magnesium (aspirin, magnesium-containing laxatives/antacids, lithium)
• To monitor if you are being treated with magnesium and/or calcium

Preparation and collection

There is usually no preparation for a magnesium blood test. If you are having other blood tests at the same time, you may need to fast (not eat or drink) for several hours before your test. Your provider will let you know if you need to prepare. Tell your provider about any prescription and over-the-counter medicines and/or supplements you are taking.

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. This usually takes less than five minutes.

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

Understanding results

Your health care provider will use your medical history and the results of other tests to interpret your magnesium blood test results. The amount of magnesium in your blood may be normal even though the amount stored in your body is low, because your body can take magnesium from your bones to keep blood levels stable. Ask your provider to explain what your test results mean.

• Low magnesium level (hypomagnesemia): Low magnesium levels may mean that: you are not getting enough magnesium in your diet (more likely in older adults and people with alcohol use disorder or malnutrition); your body is not able to absorb magnesium even if you eat the right amount (causes include digestive disorders such as Crohn's disease or taking certain medicines); your body is getting rid of too much magnesium (causes include kidney problems, long-term diarrhea, and diabetes with high blood glucose). Other health conditions linked to low magnesium include underactive parathyroid glands and pregnancy. Limitation: The amount of magnesium in your blood may be normal even though the amount stored in your body is low. Your provider may order a magnesium in urine test or a magnesium red blood cell (RBC) test for more information. Next step: Treatment for abnormal magnesium levels will depend on your condition. Your provider will discuss next steps with you.
• High magnesium level (hypermagnesemia): Higher than normal magnesium levels may mean that: you are getting too much magnesium from dietary supplements, laxatives, antacids, or certain prescription medicines (it is rare to get too much from food alone); your body has a problem getting rid of magnesium (the most common cause is kidney failure). Other health conditions linked to high magnesium include dehydration, hypothyroidism, Addison disease, and overactive parathyroid glands. Limitation: The amount of magnesium in your blood may be normal even though the amount stored in your body is low. Your provider may order a magnesium in urine test or a magnesium red blood cell (RBC) test for more information. Next step: Treatment for abnormal magnesium levels will depend on your condition. Your provider will discuss next steps with you.

• The amount of magnesium in your blood may be normal even though the amount stored in your body is low. Your provider may order a magnesium in urine test or a magnesium red blood cell (RBC) test for more information.
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• Magnesium Blood Test
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• Treatment for abnormal magnesium levels will depend on your condition. Your provider will discuss next steps with you.

What to do next

Treatment for abnormal magnesium levels will depend on your condition. Your provider will discuss next steps with you.

• What do my magnesium blood test results mean in my specific situation?
• Do I need any follow-up tests, such as a magnesium urine test or magnesium RBC test?
• What conditions could be causing my abnormal magnesium level?
• Should I change my diet or any medicines I am taking?

Evidence source

U.S. National Library of Medicine: Magnesium Blood Test