What Is the Vitamin D Test?

Vitamin D helps the body absorb calcium and maintain strong bones. With a few 10 to 15 minute sessions of sunshine UV per week, the body can produce all the vitamin D it needs. You also get vitamin D from some food sources and vitamin D is available as a dietary supplement.

Vitamin D must be converted in the liver into a chemical called 25-hydroxyvitamin D or calcidiol. The 25-hydroxyvitamin D test is the best way to monitor vitamin D. The amount of 25-hydroxyvitamin D in the blood indicates how much vitamin D the body has. The test tells you if your levels are too high or too low.

Purpose

Your healthcare provider will request a 25-hydroxyvitamin D test if they suspect you have too much or too little vitamin D. A 25-hydroxyvitamin D test may also help monitor your risk for vitamin D deficiency.

People at high risk for vitamin D deficiency include: 

• Breastfed infants
• Older adults
• Obese adults
• People who didn’t get much sun exposure
• People who have had gastric bypass surgery
• People who have conditions that affect the intestines making it difficult for the body to absorb nutrients, such as inflammatory bowel disease
• People with darker skin

If you have already been diagnosed with a deficiency, your healthcare provider can repeat the 25-hydroxyvitamin D test to see if your treatment is working.

Risks and Contraindications

Risks associated with the 25-hydroxyvitamin D testing are rare. However, risks associated with any routine blood test may include:

• Excessive bleeding
• Lightheadedness
• Slight chance of infection

There are no contraindications for this test.

Before the Test

Timing

The time to allow for this test depends on the time it takes to get your blood drawn. The blood draw itself should take only about five minutes, but you may have to wait for the phlebotomist to be available. Typically, you will not get your test results for several hours, so you will not wait at the healthcare provider's office or lab for the results.

Location

You can get a 25-hydroxyvitamin D blood test at your healthcare provider’s office, at a laboratory, or using an in-home test. At-house tests are available for purchase at pharmacies, and supermarkets. They use a finger prick sample and are likely to be less accurate than the test performed on a blood sample drawn in a tube. You perform the finger prick and send the sample to a lab to be tested.

You can also order a test online and go into a laboratory to get the blood work done. This option is uncommon.

What to Wear

You will need to bare your arm for the blood draw, so wear a short-sleeved shirt or one that is easy to roll up.

Food and Drink

You do not have to fast before this test. However, if it is being done along with other blood tests that require fasting, follow your healthcare provider's instructions as to what is appropriate timing and what you are allowed to eat or drink. Unless drinking fluids is restricted due to other tests being done, it is good to be well hydrated before a blood draw as being dehydrated can make it more difficult to find the vein.

Cost and Insurance

This test is commonly ordered when your healthcare provider suspects a problem rather than being a well-patient screening test. Your insurance may cover the cost of testing as part of diagnosing a problem, subject to any usual co-pays or deductibles. If you have ordered this test for yourself online, or purchased an at-home kit, it may not be covered by insurance.

What to Bring

Bring your insurance card, identification, and lab test orders to the site where your blood is drawn.

During the Test

Pre-Test

The 25-hydroxyvitamin D test requires a simple blood sample. A healthcare staff member (usually a nurse or health technician) will draw blood from a vein using a needle into your arm. Be sure to inform them if you have a problem with feeling faint or woozy during blood draws or if you are allergic to iodine (which is often used to sterilize the draw site).

Throughout the Test

After checking your identification, the nurse or technician will place a tourniquet on your arm and select a vein. The site will be sanitized with an antiseptic pad and the needle will be inserted. This usually hurts for a second or two.

While this test requires only a small amount of blood, one or more tubes may be drawn depending on what other tests are also ordered. Once the blood is obtained, the tourniquet is released and the draw site is covered with a sterile bandage. The tubes are labeled and sent to the lab. The blood draw process should take only about five minutes.

In children and infants, a finger prick or heel stick will provide enough blood for a sample to test.

In-home tests are easy to use. They involve pricking your finger to take a small sample of blood. The blood work is then sent away to a lab for testing. If you have requested a test online to be done at a laboratory, you will go to the laboratory at your appointment time, and someone from the lab’s staff will administer the test by drawing blood from a vein.

After the Test

You can remove the bandage on your arm after a few minutes if there isn't any ongoing bleeding. If you note bleeding, swelling, or a growing bruise, alert the medical staff or talk to your healthcare provider.

Interpreting Results

You can expect that you will wait for hours to days for the results of the test. Once your healthcare provider has results, he or she will explain the results of the test and determine if you have a vitamin D deficiency. If you used an at-home test or requested a vitamin D test through a lab, you will either get a phone call, letter, or email explaining the results.

When you get back your results, they will be in units of nanograms/milliliter (ng/mL). For example, anything around 20 ng/mL is considered adequate for bone and overall health in generally healthy people, according to the Office of Dietary Supplements. These measurement units are used by medical professionals in the United States.

There are ranges used to determine a person’s vitamin D status. These are:

• Deficient: This range is applied if a person is severely lacking vitamin D.
• Insufficient: If a person’s vitamin D range is insufficient, it means they are mildly lacking in vitamin D.
• Sufficient: Sufficient range means a person is getting enough vitamin D.

A person’s vitamin D results will depend on a person’s age or sex. The testing method and the laboratory method may also cause some variation in results.

Follow-Up

Vitamin D deficiency or toxicity can cause very serious health problems. Your healthcare provider will recommend a vitamin D supplement or other treatment if you are very deficient. Your practitioner will want you to reduce or stop supplement intake if your level is too high.

Deficient or insufficient levels of vitamin D could mean:

• You aren’t eating a well-balanced diet.
• Your intestines aren’t properly digesting vitamin D.
• You aren’t spending enough time in the sun to absorb enough vitamin D.

If you are experiencing bone pain in addition to having low vitamin D, your healthcare provider may want to do a bone density scan. This scan is painless and looks at your bone health. Vitamin D deficiency is also a risk factor for autoimmune diseases, certain cancers, and heart disease.

Your vitamin D may be too high if you are taking too many vitamins and nutritional supplements. Taking in too much vitamin D can put you at risk for liver and kidney problems. You will not get high levels from your diet or sun exposure.

A Word From Verywell

Research from the Mayo Clinic finds up to 50% of Americans may be vitamin D deficient, with older adults being at the highest risk. The results of the 25-hydroxyvitamin D test will show whether you are getting enough vitamin D. 

You should also eat plenty of foods that contain vitamin D to keep your levels stable. Vitamin D-rich foods include fatty fish (salmon, mackerel, and tuna), beef liver, egg yolks, cheese, and vitamin D-fortified dairy and cereals.

Most adults should aim for 600-800 IU (international units) of vitamin D daily. However, some studies suggest even more, from 1,000 to 4,000 IU per day, to maintain optimal levels. Your healthcare provider is in the best position to advise you on how much vitamin D you need.