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
A blood sample is needed. For information on how this is done, see: Venipuncture
Usually you will not need to fast. However, this depends on the laboratory on the testing method used.
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
This test is done to determine if you have too much or too little vitamin D in your blood.
The normal range is 30.0 to 74.0 nanograms per milliliter (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.
Lower-than-normal levels can be due to a vitamin D deficiency, which can result from:
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:
Weng FL, Shults J, Leonard MB, Stallings VA, Zemel BS. Risk factors for low serum 25-hydroxyvitamin D concentrations in otherwise healthy children and adolescents. Am J Clin Nutr. 2007;86(1):150-158.
Lappe JM, Travers-Gustafson D, Davies KM, Recker RR, Heaney RP. Vitamin D and calcium supplementation reduces cancer risk: results of a randomized trial. Am J Clin Nutr. 2007;85(6):1586-1591.
Specker BL, Valanis B, Hertzberg V, Edwards N, Tsang RC. Sunshine exposure and serum 25-hydroxyvitamin D concentrations in exclusively breast-fed infants. J Pediatr 1985;107(3):372-376.