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25-hydroxy vitamin D test

25-OH vitamin D test; Calcidiol; 25-hydroxycholecalciferol test

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Definition

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.

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.

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.

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.

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)

What Abnormal Results Mean

A lower-than-normal level 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

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.

Why the Test is Performed

This test is done to determine if you have too much or too little vitamin D in your blood.

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.

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