25-hydroxy vitamin D
Alternative names:
25-OH vitamin D
How the test is performed:
Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the 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 or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or 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.
Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
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 patient has kidney disease.
Calcium (Ca+2 ) and phosphate (PO4) levels are regulated mainly by PTH (parathormone) and 1,25-dihydroxy vitamin D in the body (calcitonin plays a lesser role). Vitamin D has to be activated by adding hydroxyl groups before it becomes a hormone. Vitamin D is first hydroxylated by an enzyme in the liver, which converts it to 25-hydroxy vitamin D. A second hydroxylation takes place in the kidneys.
The main function of 1,25-dihydroxy vitamin D is to increase absorption of Ca+2 and PO4 in the intestines and to amplify the effect of PTH in causing resorption of bone and reabsorption of calcium in the kidneys.
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