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Blood Test: Magnesium

What Is a Blood Test?

A blood test is when a sample of blood is taken from the body to be tested in a lab. Doctors order blood tests to check things such as the levels of glucose, hemoglobin, or white blood cells. This can help them find problems like a disease or medical condition. Sometimes, blood tests can help them see how well an organ (such as the liver or kidneys) is working.

What Is a Magnesium Test?

A magnesium test measures the amount of magnesium in the blood. Magnesium is a mineral that helps muscles, nerves, the immune system, and the heart work as they should. It also helps make protein and energy, and helps keep bones strong.

Why Are Magnesium Tests Done?

A magnesium test may be done if a person has signs of low magnesium levels, such as weakness, muscle cramps or twitching, or an abnormal heartbeat. It also might be done if someone has a medical condition that makes low magnesium levels more likely, such as a kidney problem.

How Should I Prepare for a Magnesium Test?

You should be able to eat and drink normally unless you're also getting other tests that require fasting beforehand. Tell your doctor about any medicines you take because some drugs might affect the test results.

It can help to wear a T shirt or other short-sleeve top on the day of the test to make things faster and easier for the technician who will be drawing the blood.

How Is a Magnesium Test Done?

Usually, the blood is drawn from a vein in your arm — most often on the inside of the elbow, but sometimes on the back of the hand. To do that, a health professional will:

  • clean the skin
  • put an elastic band (tourniquet) above the area to get the veins to swell with blood
  • insert a needle into a vein (usually in the arm inside of the elbow or on the back of the hand)
  • pull the blood sample into a vial or syringe
  • take off the elastic band and remove the needle from the vein

It's best to try to relax and stay still during the procedure since tensing muscles can make it harder and more painful to draw blood. And if you don't want to watch the needle being inserted or see the blood collecting, you don't have to. Look the other way and maybe relax by focusing on saying the alphabet backwards, doing some breathing exercises, thinking of a place that makes you happy, or listening to your favorite music.

How Long Does a Magnesium Test Take?

Most blood tests take just a few minutes. Occasionally, it can be hard to find a vein, so the health professional may need to try more than once.

What Happens After a Magnesium Test?

The health professional will remove the elastic band and the needle and cover the area with cotton or a bandage to stop the bleeding. Afterward, there may be some mild bruising, which should go away in a few days.

When Are Magnesium Test Results Ready?

Blood samples are processed by a machine, and it may take a few hours to a day for the results to be available. If the test results show signs of a problem, the doctor might order other tests to figure out what the problem is and how to treat it.

Are There Any Risks From Magnesium Tests?

A magnesium test is a safe procedure with minimal risks. Some people might feel faint or lightheaded from the test. A few teens have a strong fear of needles. If you're anxious, talk with the doctor before the test about ways to make the procedure easier.

A small bruise or mild soreness around the blood test site is common and can last for a few days. Get medical care if the discomfort gets worse or lasts longer.

If you have questions about the magnesium test, speak with your doctor or the health professional doing the blood draw.

Reviewed by: Larissa Hirsch, MD
Date reviewed: January 2018
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