A magnesium blood test measures the amount of magnesium, an important mineral, in your blood. Your body needs magnesium from your diet in order for your muscles, nerves, heart and immune system to function properly. Magnesium also helps control blood pressure and blood sugar and helps build strong bones.
Your kidneys, bones, intestines and hormones regulate the amount of magnesium in your blood. However, many conditions can change magnesium levels in the blood. Detecting high or low levels of magnesium in your blood can help your doctor diagnose certain diseases and disorders.
Your healthcare provider may order a magnesium blood test to check whether your blood magnesium levels are too low or high. You may be asked to have a magnesium blood test for the following reasons:
To investigate symptoms
If you have low magnesium levels, also known as magnesium deficiency or hypomagnesemia, your symptoms may include:
- Loss of appetite
- Nausea
- Vomiting
- Muscle cramps
- Numbness or tingling
- Fatigue
- Weakness
- Irregular heartbeat (arrhythmia)
- Attacks
If your magnesium is high, also known as hypermagnesemia, you may have the same symptoms of low magnesium as the following:
- diarrhea
- Stomach cramps
- Cardiac arrest (when the heart suddenly stops beating)
To track the effects of the condition
Your healthcare provider may also use this test to help monitor your health if you have a condition associated with abnormal magnesium levels. This may include:
- Alcohol use disorder
- Chronic kidney disease
- Chronic diarrhea
- diabetes
- Malnutrition due to eating disorders or other causes
- Low levels of potassium or calcium
To monitor the effects of the drug
If you take certain medications that can affect your magnesium levels, your healthcare provider may order a magnesium blood test to monitor your levels. These medications include:
- Antibiotics
- Aspirin
- Diuretics
- Laxatives
- lithium
- Proton pump inhibitors
- Supplements of magnesium or calcium
To determine why other mineral levels are abnormal
Magnesium plays a key role in maintaining the balance and proper functioning of other minerals in the body, such as calcium, potassium and phosphorus. It helps regulate processes such as calcium metabolism and potassium transport across cell membranes.
If your levels of these other minerals are abnormal, your healthcare provider may order a magnesium blood test to see if magnesium levels are playing a role.
You usually don’t need to prepare for a magnesium blood test in any special way.
This test is sometimes done with other blood tests that require fasting, so you may need to avoid eating and drinking for a certain amount of time in those cases. Your healthcare provider can tell you if you need to fast.
Tell your doctor about all supplements and medications you are taking. They may tell you to change your regimen before the test.
Your healthcare provider will take a blood sample from a vein in your arm. That sample is sent to a laboratory for analysis.
To take the sample, your doctor will tie a band tightly around your arm to temporarily stop the blood flow, making it easier for them to find the vein. They will sterilize the area where they plan to insert the needle, usually with rubbing alcohol. They will then insert a small needle into your vein and draw blood into a tube or vial.
You will need to stay still while your blood is drawn. You may feel a slight sting when the needle is inserted and removed. After the needle is removed, your healthcare provider will tell you to apply pressure to the site where the needle was inserted before placing a bandage over the site. The entire test usually takes less than five minutes.
After giving a blood sample for the test, you can return to normal activities. You may feel some discomfort where the needle was inserted, but it should go away soon.
There is little risk to the magnesium blood test. As with any blood test, you may experience slight discomfort or bruising where the needle is inserted.
Some people may feel faint when their blood is drawn. If you ever pass out during a blood test, tell the person taking your blood beforehand. They might make you lie down as a precaution.
Your results should be available in a few days. Your healthcare provider will contact you with your results or send them through the patient portal.
Interpreting your results
A normal level of magnesium in the blood is usually between 0.75-0.95 millimoles.
A variety of factors can affect magnesium levels. For low or high magnesium levels, your healthcare provider will consider these levels along with your medical history and other tests to make a diagnosis.
Low magnesium levels
Low magnesium levels generally mean that you’re not getting enough magnesium from food, or your body can’t absorb the mineral even if you’re getting enough of it. Low magnesium levels can be caused by conditions such as:
- Alcohol use disorder: Problems stopping or controlling alcohol use
- Chronic diarrhea: Loose or watery stools that occur three or more times a day for four weeks or longer
- Hyperaldosteronism: When the adrenal glands produce too much of the hormone aldosterone
- malnutrition: When the body does not get enough nutrients for proper functioning
- Pancreatitis: Inflammation of the pancreas
- ulcerative colitis: A condition that causes inflammation of the lining of the colon and rectum
- Uncontrolled diabetes: Complications from high blood sugar, leading to damage to your kidneys and nervous and cardiovascular systems
High levels of magnesium
Higher magnesium levels can mean you’re getting too much of the mineral from medications (such as laxatives and antacids), supplements, and food. It is rare to get too much magnesium from food alone.
Another cause of high magnesium is a medical condition that interferes with your body’s ability to regulate magnesium levels. Conditions that can lead to high magnesium levels include:
- Kidney failure: The most common condition that causes high magnesium levels, because magnesium is mainly regulated by your kidneys
- Adrenal insufficiency: Adrenal glands do not produce enough hormones
- dehydration: Losing too much fluid from your body, which can affect kidney function and reduce the amount of magnesium that is removed from the body
A magnesium blood test detects the level of magnesium in your blood. Magnesium is a mineral involved in many vital functions, and your kidneys and hormones are primarily responsible for maintaining healthy levels of magnesium in your body.
If a magnesium blood test shows that your levels are too high or too low, it could be a sign of a medical condition that needs treatment — possibly related to your kidneys or hormones.