A tuberculin skin test is done to see if you have ever had tuberculosis (TB). The test is done by putting a small amount of TB protein (antigens) under the top layer of skin on your inner forearm. If you have ever been exposed to the TB bacteria (Mycobacterium tuberculosis), your skin will react to the antigens by developing a firm red bump at the site within 2 days.
The TB antigens used in a tuberculin skin test are called purified protein derivative (PPD). A measured amount of PPD in a shot is put under the top layer of skin on your forearm. This is a good test for finding a TB infection. It is often used when symptoms, screening, or testing, such as a chest X-ray, show that a person may have TB.
A tuberculin skin test cannot tell how long you have been infected with TB. It also cannot tell if the infection is latent (inactive) or is active and can be passed to others.
A tuberculin skin test is done to find people who have tuberculosis (TB), including:
A tuberculin skin test should not be done for people:
Before having a tuberculin skin test, tell your doctor if you:
Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form (What is a PDF document?).
For a tuberculin skin test, you sit down and turn the inner side of your forearm up. The skin where the test is done is cleaned and allowed to dry. A small shot of the TB antigen (purified protein derivative, or PPD) is put under the top layer of skin. The fluid makes a little bump (wheal) under the skin. A circle may be drawn around the test area with a pen. See a picture of a person receiving a Mantoux test.
Do not cover the site with a bandage. You must see your doctor 2 to 3 days after the test to have the skin test checked.
You may feel a quick sting or pinch from the needle.
There is a very slight risk of having a severe reaction to the tuberculin skin test, especially if you have had tuberculosis (TB). An allergic reaction can cause a lot of swelling and pain at the site. A sore may be present.
You cannot get a TB infection from the tuberculin skin test, because no live bacteria are used for the test.
Some redness at the skin test site is expected. The site may itch, but it is important that you do not scratch it, since this may cause redness or swelling that could make it hard to read the skin test. If itching is a problem, put a cold washcloth on the site and then dry it.
A strong positive reaction may cause mild pain. Talk to your doctor if you have:
A tuberculin skin test is done to see if you have ever had tuberculosis (TB) (infection with Mycobacterium tuberculosis).
Redness alone at the skin test site usually means you have not been infected with TB bacteria. A firm red bump may mean you have been infected with TB bacteria at some time. The size of the firm bump (not the red area) is measured 2 to 3 days after the test to determine the result. Your doctor will consider your chance of having TB when looking at the skin test site. See a picture of the measurement of a tuberculin skin test result.
Three levels of risk have been defined:
A positive reaction usually remains visible for about 1 week.
|Normal (negative results):|
No firm bump forms at the test site, or a bump forms that is smaller than 5 mm (0.2 in.).
|Abnormal (positive results):|
A firm bump that is 5 mm (0.2 in.) in size shows a TB infection in people who are in a high-risk group.
A firm bump that is 10 mm (0.4 in.) in size shows a TB infection in people who are in a moderate-risk group.
A firm bump that is 15 mm (0.6 in.) in size shows a TB infection in people who are in a low-risk group.
A positive tuberculin skin test does not mean you have a contagious (active) infection. The test cannot tell if the infection is active or inactive (latent TB). It also can not tell the difference between a TB infection and a TB vaccination (BCG vaccination). More tests—such as a chest X-ray, a sputum culture, or both—are usually done to see if you have an active TB infection.
Reasons you may not be able to have the test or why the results may not be helpful include:
Other Works Consulted
- American Thoracic Society, Centers for Disease Control and Prevention, Infectious Diseases Society of America (2003). Treatment of tuberculosis. American Journal of Respiratory and Critical Care Medicine, 167(4): 603–662.
- Centers for Disease Control and Prevention (2005). Guidelines for using the QuantiFERON®-TB test for diagnosing latent Mycobacterium tuberculosis infection. MMWR, 54(RR-15): 49–55.
- Division of Tuberculosis Elimination, National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention (2009). Questions and Answers About TB. Available online: http://www.cdc.gov/tb/publications/faqs/default.htm.
- Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
|Primary Medical Reviewer||E. Gregory Thompson, MD - Internal Medicine|
|Specialist Medical Reviewer||R. Steven Tharratt, MD, MPVM, FACP, FCCP - Pulmonology, Critical Care Medicine, Medical Toxicology|
|Last Revised||April 15, 2011|
Last Revised: April 15, 2011
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