Alternative Names: 25-OH vitamin D

How the test is performed:

Adult or child:

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to fill with blood. A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. The tourniquet is then removed to restore circulation. After blood has been collected the needle is removed, and the puncture site is covered to stop any bleeding.

Infant or young child:

The area is cleansed with antiseptic and punctured with a sharp needle and collected in a small container. A bandage may be applied to the puncture site if there is any continued bleeding.

How to prepare for the test:

Fast for 4 hours before the test.

How the test will feel:

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.

Why the test is performed:

This test is used to determine if a person has a deficiency or excess of vitamin D.

25-hydroxy vitamin D is stored in the liver, where it is converted from other forms of vitamin D. These other forms are�either obtained from food or made in the skin by exposure to sunlight. 25-hydroxy vitamin D is changed to its active form, 1,25-dihydroxy vitamin D in the kidney. This active form of vitamin D keeps normal levels�of calcium and phosphate in the blood by acting primarily on the intestines, kidneys, and bones.

Normal Values:

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

Note: ng/mL = nanograms per milliliter

What abnormal results mean:

Low 25-hydroxy vitamin D levels indicate a deficiency in vitamin D.�Vitamin D deficiency can result�from lack of exposure to sunlight, lack of adequate vitamin D in the diet, liver and kidney diseases, malabsorption, and medications such as phenytoin, phenobarbital, and rifampin. Vitamin D deficiency may lead to low blood calcium levels (hypocalcemia), thin or weak bones (osteoporosis�and osteomalacia), and high levels of parathyroid hormone (secondary hyperparathyroidism).

High 25-hydroxy vitamin D indicates excess vitamin D (hypervitaminosis D) and is usually caused by too much vitamin D in the form of doctor-prescribed dietary supplements. This can lead to high calcium levels (hypercalcemia).

What the risks are:

• Excessive bleeding
• Fainting or feeling lightheaded
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Special considerations:

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.