The 25-hydroxy vitamin D test is the most accurate measure of the amount of vitamin D in the body.

In the kidney, 25-hydroxy vitamin D changes into an active form (called 1,25-dihydroxy vitamin D). The active form helps control blood levels of calcium and phosphate.

This article discusses the blood test used to measure the amount of 25-hydroxy vitamin D.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

Do not eat for 4 hours before the test.

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 a vitamin D deficiency or excess.

A deficiency means you do not have enough vitamin D in your body.

An excess means you have too much vitamin D in your body.

The normal range is 16.0 to 74.0 nanograms per milliliter (ng/mL). Normal value ranges may vary slightly among different laboratories.

Lower-than-normal levels suggest a vitamin D deficiency. This condition can result from:

• Lack of exposure to sunlight
• Lack of adequate vitamin D in the diet
• Liver and kidney diseases
• Malabsorption
• Certain medicines, including phenytoin, phenobarbital, and rifampin

A vitamin D deficiency may lead to:

• Low blood calcium levels (hypocalcemia)
• Thin or weak bones (rickets, osteoporosis and osteomalacia)
• High levels of parathyroid hormone (secondary hyperparathyroidism)

Higher-than-normal levels suggest excess vitamin D, a condition called hypervitaminosis D. It is usually caused by vitamin D in the form of doctor-prescribed dietary supplements. This can lead to high calcium levels (hypercalcemia).

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)

Low vitamin D levels are more common in African American children, particularly in the winter, as well as in infants who are exclusively breastfed. Low vitamin D levels have also been associated with an increased risk of developing cancer.

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):372-6.