Magnesium is a metal that is found in small amounts in every cell of your body.

What is being tested?

Magnesium is essential to many processes in your body, such as:

• producing energy from food
• enabling your muscles and nerves to work properly
• helping your cells absorb potassium and calcium

Most of the magnesium in your body comes from the food you eat. Foods high in magnesium include green vegetables (like spinach and peas), nuts and seeds, whole grains, lentils, chickpeas, beans, and some shellfish. 

About half the magnesium in your body is in your bones. The rest can be found throughout your body. Only about 1 part in 100 is in your blood.

Most people get enough magnesium from their diet, so talk to your doctor before taking magnesium supplements.

Why would I need this test?

Your doctor might want to do a blood test for magnesium if you:

• drink a lot of alcohol
• have an eating disorder or a poor diet
• have diabetes
• have a problem with your kidneys
• are taking magnesium or calcium supplements
• have too much, or too little, calcium or potassium in your blood

The signs and symptoms of not having enough magnesium in your blood include:

• tiredness
• nausea and diarrhoea
• muscle weakness, twitching muscles or muscle cramps
• extreme irritability
• irregular heartbeat
• weight loss
• seizures

If you have low levels of magnesium in your blood, it could mean:

• you are not getting enough magnesium in your diet
• your intestines are not absorbing enough magnesium
• your kidneys are excreting too much of it

How to prepare for the test

You don’t need to prepare for a blood test for magnesium.

Understanding your results

Your doctor is the best person to talk to about your blood test results. You can discuss what they mean and what comes next.

If you are pregnant, your magnesium levels might be naturally low in the second or third month of your pregnancy. Whether you are pregnant or not, a result outside the normal range might not mean that you have a health problem.

More information

• Read healthdirect's Guide to blood testing.
• Pathology Tests Explained has more information on blood tests in general and magnesium in particular.
• Healthdirect's Question Builder can help you prepare for a doctor's appointment.