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25-hydroxy vitamin D test
25-OH vitamin D test; Calcidiol; 25-hydroxycholecalciferol test
The 25-hydroxy vitamin D test is the most accurate way to measure how much vitamin D is in your body.
Vitamin D helps control calcium and phosphate levels in the body.
How the Test is Performed
A blood sample is needed.
Blood sample
Venipuncture is the collection of blood from a vein. It is most often done for laboratory testing.
How to Prepare for the Test
Usually, you will not need to fast. But, this depends on the laboratory and the testing method used. So, follow any instructions for not eating before the test.
How the Test will Feel
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.
Normal Results
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
A lower-than-normal level can be due to a vitamin D deficiency , which can result from:
Vitamin D deficiency
Rickets is a disorder caused by a lack of vitamin D, calcium, or phosphate. It leads to softening and weakening of the bones.
- 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
A low vitamin D level is more common in African-American children (especially in the winter), as well as in infants who are breastfed only. A low vitamin D level may be associated with an increased risk of developing cancer.
A higher-than-normal level may be due to excess vitamin D, a condition called hypervitaminosis D . This is most commonly caused by taking too much vitamin D. It can result in too much calcium in the body ( hypercalcemia ). This leads to many symptoms and kidney damage.
Hypervitaminosis D
Hypervitaminosis D is a condition that occurs after taking very high doses of vitamin D.
Hypercalcemia
Hypercalcemia means you have too much calcium in your blood.
Risks
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)
References
Bouillon R. Vitamin D: from photosynthesis, metabolism, and action to clinical applications. In: Jameson JL, De Groot LJ, de Kretser DM, et al, eds. Endocrinology: Adult and Pediatric . 7th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 59.
Chernecky CC, Berger BJ. Vitamin D (Cholecalciferol) – plasma or serum. In: Chernecky CC, Berger BJ, eds. Laboratory Tests and Diagnostic Procedures . 6th ed. Philadelphia, PA: Elsevier Saunders; 2013:1182-1183.
LeFevre ML; US Preventive Services Task Force. Screening for vitamin D deficiency in adults: US Preventive Services Task Force recommendation statement. Ann Intern Med . 2015;162(2):133-140. PMID 25419853. www.ncbi.nlm.nih.gov/pubmed/25419853 .
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Blood test - illustration
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. Preparation may vary depending on the specific test.
Blood test
illustration
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Blood test - illustration
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. Preparation may vary depending on the specific test.
Blood test
illustration
Review Date: 5/2/2016
Reviewed By: Brent Wisse, MD, Associate Professor of Medicine, Division of Metabolism, Endocrinology & Nutrition, 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.
