This test checks your vitamin D status mainly looking for deficiency of this vitamin. It is used if you are at high risk of vitamin D deficiency because of your lifestyle, if you have bone disease such as osteoporosis, or if you have already had test results that show you have abnormal calcium, phosphorus, and/or parathyroid hormone levels in your blood.

Vitamin D is essential for healthy bones and teeth.

There are two forms of vitamin D that are important for nutrition. These are vitamin D2 and vitamin D3. Vitamin D2 mainly comes from fortified foods like breakfast cereals, milk, and other dairy foods. Vitamin D3 is mostly made by the body when your skin is exposed to sunlight. It is also found in some foods, including eggs and fatty fish.

Both vitamin D2 and vitamin D3 are converted in the liver into 25-hydroxy-vitamin D. This is the main active form of vitamin D stored in the body. When your doctor asks for a vitamin D level to be measured in your blood sample, this is the form the laboratory will measure. The test for 25-hydroxy-vitamin D is used to check that you have an adequate supply of the vitamin.

The test is used to check for vitamin D deficiency:

• As part of a general health check, especially for anyone who may be at high risk of a vitamin D deficiency because they have limited sun exposure.
• If someone has symptoms of a bone disease such as osteoporosis and/or blood tests show abnormal levels of calcium, phosphate, and/or magnesium.
• When there are signs of bone malformation in children (rickets) and bone weakness, softness, or fracture in adults (osteomalacia).
• When someone has a malabsorption disorder. This can occur after bariatric surgery or in conditions such as cystic fibrosis or Crohn's disease that prevent fat being absorbed from the intestine.
• If someone has kidney disease 
• If the parathyroid gland is not working properly. Parathyroid hormone (PTH) is essential for vitamin D activation and there is an inverse relationship between vitamin D and PTH levels.
• To monitor someone who is taking vitamin D supplements.

Certain adult groups are at increased risk of vitamin deficiency:

• People who have low or no exposure to the sun through being housebound or confined indoors for long periods, such as the elderly and hospitalised patients, wearing head to toe clothing such as for cultural reasons, working in enclosed environments, for example factory and office workers, or night-shift workers.
• People with naturally dark skin. This is because the melanin in dark skin doesn't absorb as much UV radiation as those with fair skins and their bodies are not able to make as much vitamin D.
• Those with fair skin who avoid sun exposure because they are at risk of skin cancer.
• Someone with a disability or chronic disease that affects vitamin D metabolism.
• People who are obese.

Sample

Blood

Any preparation?

None

Reading your test report

Your results will be presented along with those of your other tests on the same form.  You will see separate columns or lines for each of these tests.

The Australian and New Zealand Bone and Mineral Society (ANZBMS) and Osteoporosis Australia OA) guidelines (2012) classify vitamin D results as:

Vitamin D levels should be higher at the end of summer to allow for a decrease in levels over winter. Your doctor may recommend a vitamin D supplement if your vitamin D level is below this target level.

Low blood levels of 25-hydroxy-vitamin D may mean:

• you are not getting enough exposure to sunlight or enough dietary vitamin D to meet your body's requirements,
• there is a problem with its absorption from the intestines, or 
• not enough is being converted to 25-hydroxy-vitamin D in the liver, which means that it is not making it into the bloodstream.

High blood levels are usually caused by taking too many vitamin pills or other nutritional supplements and can lead to calcification and damage of organs, such as the kidneys. As the body tries to lower blood calcium levels calcium phosphate compounds can be deposited into organs.

Reference intervals

Your results will be compared to reference intervals (sometimes called a normal range). 

• Reference intervals are the range of results expected in healthy people 
• When compared against them your results may be flagged high or low if they sit outside this range
• Many reference intervals vary between labs so only those that are standardised or harmonised across most laboratories are given on this website.

If your results are flagged as high or low this does not necessarily mean that anything is wrong. It depends on your personal situation. Your results need to be interpreted by your doctor.

Tell your doctor about medicines, vitamins, or supplements you are taking, because they may affect your test results.
Occasionally, drugs used to treat seizures, particularly phenytoin (Dilantin), can interfere with the liver's production of 25-hydroxy-vitamin D.