Tuberculin Skin Test
A tuberculin skin test (also called a Mantoux tuberculin test) is done to see if you have ever been exposed to 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 if you have active TB that can be passed to others.
Why It Is Done
A tuberculin skin test is done to find people who have tuberculosis (TB), including:
- People who have been in close contact with someone known to have TB.
- Health care workers who are likely to be exposed to TB.
- People with TB symptoms, such as an ongoing cough, night sweats, and unexplained weight loss.
- People who have had an abnormal chest X-ray.
- People who have had a recent organ transplant or have an impaired immune system, such as those with human immunodeficiency virus (HIV).
A tuberculin skin test should not be done for people:
- With a known TB infection.
- Who have had a previous severe reaction to the TB antigens.
- Who have a skin rash that would make it hard to read the skin test.
How To Prepare
In general, there's nothing you have to do before this test, unless your doctor tells you to.
How It Is Done
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 tuberculosis 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.
There is a very slight risk of having a severe reaction to the tuberculin skin test, especially if you've had tuberculosis (TB). An allergic reaction can cause a lot of swelling and pain at the site. You may have a sore.
You cannot get a TB infection from the tuberculin skin test. That's because no live bacteria are used for the test.
A tuberculin skin test is safe for women who are pregnant or who breastfeed.
Redness alone at the skin test site usually means you haven't 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 find out the result. Your doctor will consider your chance of having TB when looking at the skin test site.
Results of the test depend on your risk for TB. If you are in a high-risk group, a smaller bump is considered a sign of infection. People at low risk for having TB need to have a larger bump to be diagnosed with a TB infection.
Three levels of risk have been defined:
- High-risk group.
This includes people who have HIV, those who have had close recent contact with a person who has active TB, and those who have symptoms or a chest X-ray that shows TB. Other people at high risk for TB include those who take corticosteroid medicines for a long time and people who take biologics. (These are used to treat rheumatoid arthritis and Crohn's disease.)
- Moderate-risk group.
This includes people who have recently moved from or traveled in a country with a high rate of TB; those who use illegal drugs by injection (intravenous drug users); people who live in nursing homes; workers in hospitals, nursing homes, schools, and prisons; children younger than 4 years old; children ages 4 to 18 who are exposed to high-risk adults; and homeless people. Others at moderate risk for having TB include people who are 10% or more below their ideal body weight and people who have kidney failure, diabetes, leukemia, or cancer or who've had part of their stomach removed (gastrectomy).
- Low-risk group.
This includes people who don't have any possible exposure to TB listed in the other risk groups.
A positive reaction can usually be seen 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 suggests a TB infection in people who are in a high-risk group.
A firm bump that is 10 mm (0.4 in.) in size suggests a TB infection in people who are in a moderate-risk group.
A firm bump that is 15 mm (0.6 in.) in size suggests a TB infection in people who are in a low-risk group.
A positive tuberculin skin test doesn't mean you have contagious (active) TB. The test can't tell whether you have active or inactive (latent) TB. It also can't 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.
Current as of: September 23, 2020
Author: Healthwise Staff
E. Gregory Thompson MD - Internal Medicine
Adam Husney MD - Family Medicine
Kathleen Romito MD - Family Medicine
R. Steven Tharratt MD, MPVM, FACP, FCCP - Pulmonology, Critical Care Medicine, Medical Toxicology
W. David Colby IV MSc, MD, FRCPC - Infectious Disease
Current as of: September 23, 2020
Author: Healthwise Staff
Medical Review:E. Gregory Thompson MD - Internal Medicine & Adam Husney MD - Family Medicine & Kathleen Romito MD - Family Medicine & R. Steven Tharratt MD, MPVM, FACP, FCCP - Pulmonology, Critical Care Medicine, Medical Toxicology & W. David Colby IV MSc, MD, FRCPC - Infectious Disease
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