What is a vitamin D blood test?

Vitamin D is a nutrient that is important for healthy bones, muscles and teeth. It helps your body absorb calcium. Having enough vitamin D may reduce your chance of developing many common conditions.

Vitamin D can be found in two forms:

1. Vitamin D₂ (ergocalciferol) comes from some foods (fortified cereals, dairy, eggs and fish).
2. Vitamin D₃ (cholecalciferol) is made by your skin, when it is exposed to sunlight.

Vitamin D₂ and vitamin D₃ need to be turned into the hormone 25-hydroxy-vitamin D to function effectively in your body. This is done by your liver and kidneys.

The vitamin D blood test measures how much 25-hydroxy-vitamin D is available in your body.

Other forms of vitamin D can also be measured if needed.

If you do not have enough vitamin D, your doctor may recommend you take a supplement.

When is a vitamin D blood test used?

A vitamin D test might be part of a check-up for people at risk of having low vitamin D levels (deficiency), such as people who:

• have symptoms of vitamin D deficiency
• are not exposed to a lot of sun
• take certain medicines

Your doctor might also recommend this test if you have:

• abnormal levels of minerals such as calcium, phosphate or magnesium in your blood
• problems or weakness in your bones (such as osteoporosis or rickets)
• a health condition that might result in, or be caused by, too much or too little vitamin D, for example:
  • cystic fibrosis
  • Crohn's disease
  • kidney disease
• problems with your parathyroid gland

If you have low vitamin D levels and a young baby, your doctor may refer your child for a blood test to check their levels.

Where can I get a vitamin D blood test?

You usually need to go to a pathology collection centre to get a vitamin D blood test. Your doctor will recommend one near you. You need a referral to get this blood test.

Sometimes, your doctor will collect blood for testing in their clinic. Blood tests are also routinely performed in hospitals.

How do I prepare for a vitamin D blood test?

No special preparation is required for a vitamin D blood test.

If you are having other blood tests done at the same time, there may be special instructions you need to follow. It's best to check with your doctor or the pathology collection centre. Ask them if there is anything you need to do before having your blood test.

What happens during a vitamin D blood test?

A health professional will take a sample of blood from a vein in your arm. The blood will then be sent to a laboratory for testing.

What do my vitamin D blood test results mean?

The 'normal range' for vitamin D levels in the general population is not yet fully defined, and different laboratories may show varying normal ranges.

A low level of 25-hydroxy-vitamin D may mean:

• you are not getting enough vitamin D from sunlight or from food
• you have problems with vitamin D absorption from your intestines
• your liver is not making enough of this type of vitamin D

A high level of vitamin D could mean:

• you are taking too much of a vitamin D supplement
• you have a parathyroid problem or another medical condition

Vitamin D levels can be higher in summer than in winter.

Talk to your doctor about what your results mean for you.

Resources and support

About Vitamin D testing

Go to the Pathology Tests Explained website for more information about Vitamin D testing.

Read the Royal Australian College of General Practitioners' (RACGP) patient fact sheet on Vitamin D testing.

About bone health

Visit Healthy Bones Australia to learn about bone health and osteoporosis.

About blood testing

See healthdirect's 'Guide to blood testing' to learn more about blood tests in general including information such as:

• what to consider before having the test
• what happens during a blood test
• the accuracy of your results
• the costs of your blood tests

You can also call the healthdirect helpline on 1800 022 222 (known as NURSE-ON-CALL in Victoria). A registered nurse is available 24 hours a day, 7 days a week.'