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Vitamin D Tests

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Also known as: Ergocalciferol (Vitamin D2); Cholecalciferol (Vitamin D3); Calcidiol (25-hydroxyvitamin D[25(OH)D]); Calcifidiol (25-hydroxy-vitamin D); Calcitriol (1,25 dihydroxy-vitamin D [1,25(OH)(2)D])
Formal name: 25-hydroxyvitamin D; 1,25 dihydroxyvitamin D

The Test Sample

What is being tested?

Vitamin D is a family of compounds that is essential for the proper growth and formation of teeth and bones. This test measures the level of vitamin D in the blood.

Two forms of vitamin D can be measured in the blood, 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D. The 25-hydroxyvitamin D is the major form found in the blood and is the relatively inactive precursor to the active hormone, 1,25-dihydroxyvitamin D. Because of its long half-life and higher concentration, 25-hydroxyvitamin D is commonly measured to assess and monitor vitamin D status in individuals.

Vitamin D comes from two sources: endogenous, which is produced in the skin on exposure to sunlight, and exogenous, which is ingested in foods and supplements. The chemical structures of the types of vitamin D are slightly different, and they are named vitamin D2 (ergocalciferol, which comes from plants) and vitamin D3 (cholecalciferol, which comes from animals). The D2 form is found in fortified foods and in most vitamin preparations and supplements. Vitamin D3 is the form produced in the body and is also used in some supplements. Vitamin D2 and D3 are equally effective when they are converted by the liver and the kidney into the active form, 1,25-dihydroxyvitamin D.

Some tests do not distinguish D2 and D3 forms of the vitamin and report only the total result. Newer methods, however, may report levels of both D2 and D3 and then add them together for a total level.

The main role of vitamin D is to help regulate blood levels of calcium, phosphorus, and (to a lesser extent) magnesium. Vitamin D is vital for the growth and health of bone; without it, bones will be soft, malformed, and unable to repair themselves normally, resulting in diseases called rickets in children and osteomalacia in adults. Vitamin D has also been shown to influence the growth and differentiation of many other tissues and to help regulate the immune system. These other functions have implicated vitamin D in other disorders, such as autoimmunity and cancer.

Based on data from the National Health and Nutrition Examination Survey (NHANES), the Centers for Disease Control and Prevention (CDC) reports that two-thirds of U.S. population has sufficient vitamin D, while roughly one-quarter are at risk of inadequate vitamin D and 8% are at risk of deficiency, as defined by the Dietary Reference Intake (DRI) set by the Institute of Medicine. People at higher risk of deficiency include the elderly, people living in institutions who don't get enough sun exposure, and people with darker skin.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.