What It Is

A magnesium test is done to assess magnesium levels in the blood. Magnesium, a mineral obtained mostly from food, helps the:

• muscles and nerves function
• heart maintain normal rhythm
• bones stay strong
• body process energy and make proteins

Why It's Done

Doctors may order a magnesium test to help diagnose or monitor:

• kidney problems (whether the kidneys are excreting or retaining too much magnesium)
• gastrointestinal and nutritional conditions (problems with intestinal absorption or malnutrition)
• abnormal heart rhythm
• muscle problems, such as cramping, twitching, or weakness
• low calcium or potassium levels (because magnesium works closely with other minerals, its levels can directly affect the body's overall chemical balance)

Preparation

No special preparations are needed for this test. However, certain drugs — especially those that contain magnesium, such as some antacids and laxatives — might alter the test results, so tell your doctor about any medications your child is taking.

On the day of the test, having your child wear a short-sleeve shirt can make things easier for the technician who will be drawing the blood.

The Procedure

A health professional will usually draw the blood from a vein. For an infant, the blood may be obtained by puncturing the heel with a small needle (lancet). If the blood is being drawn from a vein, the skin surface is cleaned with antiseptic, and an elastic band (tourniquet) is placed around the upper arm to apply pressure and cause the veins to swell with blood. A needle is inserted into a vein (usually in the arm inside of the elbow or on the back of the hand) and blood is withdrawn and collected in a vial or syringe.

After the procedure, the elastic band is removed. Once the blood has been collected, the needle is removed and the area is covered with cotton or a bandage to stop the bleeding. Collecting blood for this test will only take a few minutes.

What to Expect

Either method (heel or vein withdrawal) of collecting a sample of blood is only temporarily uncomfortable and can feel like a quick pinprick. Afterward, there may be some mild bruising, which should go away in a few days.

Getting the Results

The blood sample will be processed by a machine. The results are commonly available after a few hours or the next day.

If magnesium levels are found to be either elevated or deficient, further testing may be necessary to determine what's causing the problem and how to treat it.

Risks

The magnesium test is considered a safe procedure. However, as with many medical tests, some problems can occur with having blood drawn:

• fainting or feeling lightheaded
• hematoma (blood accumulating under the skin causing a lump or bruise)
• pain associated with multiple punctures to locate a vein

Helping Your Child

Having a blood test is relatively painless. Still, many children are afraid of needles. Explaining the test in terms your child can understand might help ease some of the fear.

Allow your child to ask the technician any questions he or she might have. Tell your child to try to relax and stay still during the procedure, as tensing muscles and moving can make it harder and more painful to draw blood. It also may help if your child looks away when the needle is being inserted into the skin.

If You Have Questions

If you have questions about the magnesium test, speak with your doctor.

Reviewed by: Steven Dowshen, MD
Date reviewed: March 2011