25-hydroxy vitamin D

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Blood test

The 25-hydroxy vitamin D test measures the amount of 25-hydroxy vitamin D in the blood.

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

25-OH vitamin D

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.

Fast 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 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.

Review Date: 02/27/2006
Reviewed By: David Juan, MD, Specializing in Endocrinology Issues, Good Hand Medical Center and Alperin Medical Clinics, Santa Clara, CA. Review provided by VeriMed Healthcare Network.