Test Overview
A vitamin D test measures the amount of vitamin D in the blood. Your body needs vitamin D to absorb calcium. Calcium keeps your bones and muscles healthy and strong.
If your muscles don't get enough calcium, they can cramp, hurt, or feel weak. You may have long-term (chronic) muscle aches and pains.
If you don't get enough vitamin D throughout life, you are more likely to have thin and brittle bones (osteoporosis) in your later years.
Children who don't get enough vitamin D may not grow as much as others their age. They also have a chance of getting a rare disease called rickets.
Your body uses sunshine to make its own vitamin D. Vitamin D is found in foods such as egg yolks, liver, and saltwater fish. It is added to many food products, such as milk and cereals. You can also get it as supplements, often combined with calcium.
Many people can get the amount of vitamin D needed each day through food and sunlight.
The vitamin D test is also called the 25-hydroxy vitamin D, or 25(OH)D, test.
Why It Is Done
You may need this test if you:
- Have low levels of calcium or phosphate.
- Have osteoporosis or osteopenia.
- Had a broken bone after a fall from standing, such as if you trip and fall. The break most commonly occurs in the hip or wrist.
- Have gone through menopause and worry about osteoporosis.
- Have low levels of vitamin D.
How To Prepare
There is nothing to do to prepare for this test.
How It Is Done
Your health professional drawing blood will:
- Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
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Clean the needle site with alcohol.
- Put the needle into the vein. More than one needle stick may be needed.
- Attach a tube to the needle to fill it with blood.
- Remove the band from your arm when enough blood is collected.
- Put a gauze pad or cotton ball over the needle site as the needle is removed.
- Put pressure on the site and then put on a bandage.
How It Feels
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
Risks
There is very little chance of a problem from having a blood sample taken from a vein.
- You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
- In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
Results
A vitamin D test measures the amount of vitamin D in the blood.
The normal values listed here-called a reference range-are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.
High values
High levels of vitamin D can be caused by:
- William's syndrome. This is a genetic problem that causes growth delays before and after birth.
- Taking too many vitamin D supplements.
Low values
Low levels of vitamin D can be caused by:
- Kidney disease.
- Liver disease.
- Not getting enough sunlight.
- Not getting enough vitamin D in your diet.
What Affects the Test
Medicines that may decrease vitamin D levels include:
- Orlistat. Examples include Xenical and Alli. This is a weight-loss medicine.
- Cholestyramine. Examples include Prevalite and Questran.
- Phenytoin. Examples include Dilantin and Phenytek. This medicine prevents seizures.
What To Think About
- Vitamin D levels may change from spring and summer to fall and winter. Less sunlight in the fall and winter may cause lower vitamin D levels.
- A test with a similar name is the 1,25-dihydroxyvitamin D test. This test does not give a good measurement of vitamin D levels.
References
Citations
- Pagana KD, Pagana TJ (2011). Vitamin D (25-hydroxy vitamin D). In Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed., pp. 543-547. St. Louis: Mosby.
Credits
ByHealthwise Staff
Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerKathleen Romito, MD - Family Medicine