25-hydroxy vitamin D
The 25-hydroxy vitamin D test measures the amount of 25-hydroxy vitamin D in blood serum.
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.
Infants and children:
The preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. For general information regarding how you can prepare your child, see the following topics:
- Infant test or procedure preparation (birth to 1 year)
- Toddler test or procedure preparation (1 to 3 years)
- Preschooler test or procedure preparation (3 to 6 years)
- School age test or procedure preparation (6 to 12 years)
- Adolescent test or procedure preparation (12 to 18 years)
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 a deficiency or excess of vitamin D.
25-hydroxy vitamin D is the most accurate measure of vitamin D stores in the body.
25-hydroxy vitamin D is stored in the liver where it is converted from other forms of vitamin D either obtained from the diet or made in the skin by sunlight. 25-hydroxy vitamin D is then changed to its active form, 1,25-dihydroxy vitamin D in the kidney. This active form of vitamin D causes increases in calcium and phosphate by activity primarily on the intestines, kidney and bone.
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, either from lack of exposure to sunlight or lack of adequate vitamin D in the diet. 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
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.
by Armen E. Martirosyan, M.D.
All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.