Vitamin D Tests
- Also Known As:
- Ergocalciferol Test
- Cholecalciferol Test
- Vitamin D2 Test
- Vitamin D3 Test
- Calcidiol Test
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Test Quick Guide
Vitamin D testing measures the level of this essential substance in your blood. It is used to diagnose vitamin D deficiencies or to monitor treatment for a known deficiency.
The main function of vitamin D is to aid in the absorption of calcium during bone formation. Maintaining adequate levels of vitamin D supports healthy bones. In addition, vitamin D has anti-inflammatory and other properties that play a role in maintaining normal muscle, immune, and nervous system functions.
Vitamin D blood testing can be used to check your levels so a deficiency can be corrected. Less commonly, vitamin D testing may be used to detect vitamin D toxicity, a condition in which there is an excess of vitamin D in the body.
About the Test
Purpose of the test
Vitamin D testing is used to determine whether you have enough vitamin D in your body to support normal bodily functions. An inadequate level of vitamin D is called a deficiency. A more mild lack of vitamin D can be referred to as an insufficiency.
Your health care provider may order a vitamin D test in order to:
- Screen for a possible deficiency or insufficiency if you have a risk factor that increases the likelihood of having a low level of vitamin D. A risk factor is anything that makes it more likely for a health problem to occur.
- Diagnose a vitamin D deficiency or insufficiency if you have symptoms that suggest low levels. Testing may also be used to diagnose a rare vitamin D toxicity due to excess intake of vitamin D supplements.
- Monitor your vitamin D levels if you are being treated for a previously detected deficiency.
What does the test measure?
Vitamin D is necessary for maintaining health. It helps your body absorb minerals like calcium, magnesium, and phosphorus from your diet. Vitamin D also helps maintain proper levels of calcium, phosphate, and parathyroid hormone in your blood.
Vitamin D plays an important role in helping the body absorb calcium, which makes it crucial to maintaining bone health. Inadequate levels of vitamin D can lead to abnormal bone development, bone weakness, and pain.
Unlike other vitamins, Vitamin D can be created in the human body. When your skin is exposed to direct sunlight, ultraviolet radiation converts a chemical in your skin into an active form of vitamin D. Vitamin D can also be obtained through the diet, including from fortified or vitamin-enriched foods and vitamin supplements.
There are two major forms of Vitamin D:
- Vitamin D2 (ergocalciferol): Vitamin D2 is created in plants, such as yeast or mushrooms. It is also available as a supplement and in fortified foods.
- Vitamin D3 (cholecalciferol): Vitamin D3 is generated in the skin when it is exposed to sunlight. It is also found in some animal-based foods and may be consumed in certain fortified foods or dietary supplements.
Both vitamin D2 and D3 need to go through chemical changes before being able to be used by the body. These changes occur in the liver and the kidneys and convert vitamin D into measurable substances called 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D:
- 25-hydroxyvitamin D: In the liver, vitamin D is converted into 25-hydroxyvitamin D, which is also called 25(OH)D. Most of the time, vitamin D levels will be tested by measuring blood levels of 25(OH)D. Testing 25(OH)D is considered the most accurate way to measure how much vitamin D is in your body because 25(OH)D is the major form of vitamin D circulating in the bloodstream.
- 1,25-dihydroxyvitamin D: In the kidneys, 25(OH)D is converted into a hormone called active vitamin D or 1,25-dihydroxyvitamin D. Sometimes, health care providers may check your blood level of 1,25 dihydroxyvitamin D, which is also called 1,25(OH)2D.This substance is generally not used to detect inadequate vitamin D levels, but it may be measured in patients with abnormal calcium levels or kidney problems.
When should I get a vitamin D test?
Vitamin D testing is ordered to determine if a deficiency, insufficiency, or toxic level of vitamin D is present or to monitor treatment for a previously diagnosed deficiency.
Your health care provider may order a vitamin D blood test for you if you are experiencing symptoms of a vitamin D deficiency, such as:
- Weakening of the bones
- Abnormal bone development
- Bone deformity
- Bone pain
- Muscle weakness or cramps
- Dental abnormalities
Sometimes vitamin D tests are used as screening tests for individuals at increased risk of a vitamin D deficiency. Screening tests are conducted before any symptoms occur. The following are factors that may increase your risk of developing a vitamin D insufficiency or deficiency:
- Age over 65
- History of weight loss surgery
- Reduced ability to make vitamin D in the skin due to limited sun exposure, sunscreen use, or dark skin pigmentation
- Digestive diseases that make it difficult to absorb nutrients from food, including celiac disease and Crohn’s disease
- Kidney and liver disease
- Use of certain medications
Vitamin D tests may also be ordered if your health care provider suspects that you may have abnormally high vitamin D levels, known as vitamin D toxicity. This occurs as a result of taking too much vitamin D in supplements rather than from too much sun exposure or dietary intake.
Excess vitamin D in supplement form may cause your body to absorb more calcium from food and to reabsorb calcium from the bones into the blood. This results in excess calcium in the blood, also known as hypercalcemia, which can lead to symptoms like fatigue, confusion, bone pain, nausea and vomiting, frequent urination, and kidney problems.
A health care provider who is familiar with your medical history is in the best position to determine whether you might benefit from vitamin D testing.
Finding a Vitamin D Test
How to get tested
Vitamin D testing is typically ordered by a health care provider and takes place in a medical setting like a doctor’s office, clinic, laboratory, or hospital.
Can I take the test at home?
Several at-home test kits are commercially available that allow you to provide a blood sample for vitamin D testing. Some kits test only your vitamin D levels, and others test vitamin D in combination with additional measurements, such as other vitamins and minerals or C-reactive protein.
Vitamin D home collection kits may be purchased online and include the materials you need to take a finger prick sample of blood that is returned by mail to the testing company to analyze. Your at-home vitamin D test results are usually reported to you via a secure online platform.
At-home testing can be convenient and enable you to participate in your health care, but at-home tests cannot replace working with a health care professional. If you have symptoms of an abnormal level of vitamin D, be sure to share your concerns with your health care provider. If an at-home test detects a vitamin D deficiency, insufficiency, or toxicity, your doctor will likely recommend retesting your vitamin D level to confirm the result.
How much does the test cost?
The cost of vitamin D blood testing depends on where you have the test taken, whether other tests are performed at the same time, and whether you have health care coverage.
The costs of testing may include an office visit, a blood draw fee, and charges from the laboratory analyzing your blood sample. These costs are usually covered by insurance when the test is ordered by your health care provider. However, you may wish to check with your health insurance company to learn whether you are responsible for a deductible or copay.
Taking a Vitamin D Test
A blood sample is used to measure your vitamin D level as ordered by a health care provider. A blood sample will usually be taken from your arm in a medical setting like a doctor’s office or hospital and analyzed in a medical laboratory.
Before the test
Usually, there are no special preparations required for vitamin D blood testing. Your health care provider will tell you if any preparations are needed for your test.
During the test
Most often, a small blood sample is taken from a vein in your arm. The health care professional taking the sample may tie a band around your upper arm to make your veins easier to locate. Then they disinfect a small patch of skin with an antiseptic wipe. Next, they insert a needle attached to a collection tube into your skin and withdraw a small amount of blood.
After the test
After your blood sample has been collected and the needle is withdrawn, you may be asked to apply pressure to the site with a piece of gauze and then a bandage. This stops the site from bleeding and may prevent bruising. You will be able to resume your normal activities, including driving, immediately after the test.
There are very few risks associated with a blood draw. You may have slight bruising in the area where the blood sample was taken, especially if you do not maintain pressure on the wound for several minutes.
Vitamin D Test Results
Receiving test results
Vitamin D blood test results are usually available within a few business days. Your health care provider may call you with the results or request that you make an appointment to discuss them. You may also receive your results through a secure online patient portal or in the mail.
Interpreting test results
Your test report will include information about your level of vitamin D and the reference range used to interpret your result. Reference ranges are the test result values that are considered optimal for health. Results that fall outside the reference range may indicate a health issue.
Reference ranges can vary by laboratory. Additionally, some labs may break down your levels of vitamin D2 and D3, while other laboratories report the combined total.
Medical experts and organizations have differing opinions on the ideal levels of vitamin D. Generally, test results can be used to distinguish between an insufficiency, in which vitamin D amounts are only slightly outside of the ideal range, and a deficiency, which can cause more serious problems.
The table below summarizes the test results, interpretations, and potential medical outcomes that experts consider when looking at a patient’s 25-hydroxyvitamin D level:
|Vitamin D Test Results|
|Test Result||Interpretation||Potential Medical Outcome|
|Deficient||Vitamin D levels too low to support healthy bodily functions||There is not enough calcium in the blood. Weakening of bones or painful bone problems may occur.|
|Insufficient||Vitamin D levels lower than the ideal reference range||Most people are asymptomatic. There may be accelerated bone loss or increased risk of fracture.|
|Sufficient||Vitamin D levels fall within reference range||There is an adequate amount of vitamin D and no known risks for health problems.|
|Risk of toxicity||Excessive amount of vitamin D in the body||This can cause too much calcium to circulate in the blood, which can lead to confusion, loss of appetite, vomiting, and muscle weakness.|
Your health care provider can determine what your vitamin D test result means for your health.
Are test results accurate?
Testing 25-hydroxyvitamin D is considered the most accurate way to measure whether your intake of vitamin D, both through sun exposure and diet, is adequate.
However, a number of factors are taken into account when interpreting your vitamin D test results, including differences in tests and laboratory procedures, your weight and skin pigmentation, and medical conditions like kidney failure which can impact the body’s ability to use vitamin D.
You can talk with your health care team to learn more about the accuracy and significance of vitamin D testing in your specific case.
Do I need follow-up tests?
Not all people who have abnormal levels of vitamin D require follow-up testing. If your vitamin D test shows an abnormal result, your health care provider will determine whether you need additional testing based on several factors. These include your symptoms, the results of a physical examination, and how far outside the reference range your vitamin D level falls.
When vitamin D levels are very low, more tests may be ordered. These may include repeat vitamin D testing as well as tests of other substances, such as:
- Alkaline phosphatase (ALP)
- Parathyroid hormone (PTH)
- Blood urea nitrogen (BUN)
- Tissue transglutaminase antibodies
If your vitamin D level is abnormally high, you may require additional tests, including:
X-rays or other imaging tests may be ordered for some patients with vitamin D levels outside the reference range. Imaging tests are helpful in assessing how abnormal vitamin D levels have impacted the bones.
If you are given treatment to bring your vitamin D levels into the normal range, repeat vitamin D testing and calcium levels may be used to monitor treatment response.
Questions for your doctor about test results
You may wish to ask your doctor or other health care provider questions about your vitamin D test results, such as:
- Do my vitamin D test levels indicate a deficiency or insufficiency?
- Do my test results show that I have been taking too much vitamin D as a supplement?
- Will additional testing be needed?
- Do my test results indicate that treatment is needed?
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