25-hydroxy vitamin D test
The 25-hydroxy vitamin D test is the most accurate way to measure how much vitamin D is in your body.
In the kidney, 25-hydroxy vitamin D changes into an active form of the vitamin. The active form of vitamin D helps control calcium and phosphate levels in the body.
This article discusses the blood test used to measure the amount of 25-hydroxy vitamin D.
25-OH vitamin D test; Calcidiol; 25-hydroxycholecalciferol test
How the Test is Performed
A blood sample is needed.
How to Prepare for the Test
Usually you will not need to fast. However, this depends on the laboratory and the testing method used.
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing.
Why the Test is Performed
This test is done to determine if you have too much or too little vitamin D in your blood.
The normal range of vitamin D is measured as nanograms per milliliter (ng/mL). Many experts recommend a level between 20 and 40 ng/mL. Others recommend a level between 30 and 50 ng/mL.
The examples above are common measurements for results of these tests. Normal value ranges may vary slightly among different laboratories. Some laboratories use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results, and whether you may need vitamin D supplements.
What Abnormal Results Mean
Lower-than-normal levels can be due to a vitamin D deficiency, which can result from:
- Lack of exposure to sunlight
- Lack of enough vitamin D in the diet
- Liver and kidney diseases
- Poor food absorption
- Use of certain medicines, including phenytoin, phenobarbital, and rifampin
Low vitamin D levels are more common in African-American children (especially in the winter), as well as in infants who are breastfed only. Low vitamin D levels have also been associated with an increased risk of developing cancer. For more information, see the article on vitamin D deficiency.
Higher-than-normal levels may be due to excess vitamin D, a condition called hypervitaminosis D.
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight but may include:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
Dawson-Hughes B, Mithal A, Bonjour JP, et al. IOF position statement: vitamin D recommendations for older adults. Osteoporosis Int. 2010;21:1151-1154.
Dietary Reference Intakes for Calcium and Vitamin D: Institute of Medicine Brief Report. www.iom.edu/Reports/2010/Dietary-Reference-Intakes-for-Calcium-and-Vitamin-D/Report-Brief.aspx
Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 201;96:1911-1930.
Reviewed By: Gordon A. Starkebaum, MD, Professor of Medicine, Division of Rheumatology, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.