Vitamin D Test

What is a Vitamin D Test?

A vitamin D test measures the level of vitamin D in your blood to make sure you have enough for your body to work well. Vitamin D is essential for healthy bones and teeth. It also helps keep your muscles, nerves, and immune system working normally.

Having low levels of vitamin D is a common problem that can lead to bone disorders and other medical problems. Vitamin D testing can let you know if you need to increase your vitamin D levels.

You get vitamin D in three ways:

• Your body makes vitamin D when your bare skin is exposed to sunlight.
• You get vitamin D from certain foods. Only a few foods, such as egg yolks and fatty fish, naturally contain vitamin D. That's why vitamin D is added to foods, including breakfast cereals, milk, and other dairy items.
• You can take vitamin D supplements.

Before your body can use vitamin D, your liver must change it into another form called 25 hydroxyvitamin D, or 25(OH)D. Most vitamin D blood tests measure the level of 25(OH)D in your blood.

Your kidneys use 25(OH)D to make "active vitamin D." Active vitamin D lets your body use calcium to build bone and helps other cells work properly. If you have kidney problems or abnormal calcium levels in your blood, your health care provider may order a test of active vitamin D. But this test is not generally used to check whether you have enough vitamin D.

Testing 25(OH)D is the most accurate way to measure how much vitamin D is in your blood.

Other names: 25-hydroxyvitamin D, 25(OH)D, cholecalciferol test, ergocalciferol test, calcidiol test, vitamin D2 test, vitamin D3 test

What is it used for?

A vitamin D test is used to screen for low levels of vitamin D in your blood so you can treat it with supplements before it causes health problems.

If you have a known bone disorder or a problem absorbing calcium, a vitamin D test may be used to see if a lack of vitamin D is causing your condition. Your provider may order a Vitamin D test if you have:

• Osteomalacia, soft bones, often with muscle weakness
• Low bone density, which can lead to osteoporosis.
• Rickets, a problem with bone growth in children

Because vitamin D can affect many parts of your body, you may have your vitamin D level checked if you have other chronic (long-term) medical conditions. Ask your provider if you should be tested.

Why do I need a vitamin D test?

Your provider may order a vitamin D test if you have signs or symptoms of a bone condition that may be related to a vitamin D deficiency (very low levels of vitamin D), such as:

• Bone pain
• Muscle weakness or aches
• Soft bones
• Deformed bones
• Weak bones and fractures (broken bones)
• Low bone density (osteoporosis or osteopenia)

A vitamin D insufficiency (mildly low levels of vitamin D) usually doesn't cause symptoms. So, your provider may order a test if you have a high risk for developing a deficiency because you:

• Are older than 65. As you age, your skin is less able to make vitamin D from sunlight.
• Rarely expose your skin to sunshine because you stay indoors, cover up outside, use sunscreen, or live where there is little sunlight
• Have dark skin, which makes less vitamin D from sunlight
• Have had weight loss surgery
• Have obesity
• Have a condition that makes it difficult to absorb nutrients in food, such as Crohn's disease, ulcerative colitis, and celiac disease
• Have kidney or liver disease that affects your ability to change vitamin D into a form your body can use
• Take certain medicines that affect your vitamin D levels

A lack of vitamin D can cause serious problems for babies and children. A provider may order a screening test for:

• Babies that are mainly fed breastmilk. Breastmilk is low in vitamin D (All babies need vitamin D supplements shortly after birth, unless they are fed only formula, which contains vitamin D.)
• Children with darker skin who live in areas with little sunlight
• Children with diets low in vitamin D

If you are taking vitamin D supplements to increase your vitamin D level, your provider may order a test to see if your vitamin D levels are improving.

What happens during a vitamin D test?

A vitamin D test is a blood test. During a blood test, a health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Will I need to do anything to prepare for the test?

You don't need any special preparations for a vitamin D test.

Are there any risks to the test?

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?

Your test results may be reported in different ways. It may give you a total vitamin D result, or it may include separate results for vitamin D2 and vitamin D3. These two types of vitamin D work about the same in your body. Your total vitamin D level is the sum of these two types. The total vitamin D number is the important number.

If your total vitamin D level shows a vitamin D deficiency or insufficiency, it may mean you:

• Don't get enough vitamin D from your diet and/or exposure to sunlight
• Have trouble absorbing vitamin D in your food, which may be a sign of a malabsorption disorder
• Have trouble changing vitamin D into a form your body can use, which may be a sign of kidney or liver disease

The treatment for low vitamin D levels is usually supplements and/or dietary changes. This is usually safer than getting more sun, which may cause skin cancer.

If your total vitamin D level shows you have too much vitamin D, it is most likely from getting too much from supplements. This is very uncommon, but if it happens, you'll need to stop taking these supplements to reduce your vitamin D levels. Too much vitamin D can cause serious damage to your organs and blood vessels. If you take vitamin D supplements, ask your provider what dose is right for you.

To learn what your vitamin D test results mean, talk with your provider.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about a vitamin D test?

Be sure to tell your provider about medicines, vitamins, or supplements you are taking, because they can affect your test results.