Tuberculin Skin Test
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.
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
Before having a tuberculin skin test, tell your doctor if you:
- Have symptoms of tuberculosis (TB), such as an ongoing cough, night sweats, or unexplained weight loss.
- Have had a severe reaction to a tuberculin skin test in the past.
- Have had TB in the past.
- Have risk factors for TB, which are things that increase your risk. Risk factors for TB include:
- Contact with a person who has TB.
- A job as a health care worker that may cause you to be exposed to people with TB.
- Having lived or traveled in a country where TB is common.
- Have been given a TB vaccination. The vaccine contains a bacteria called BCG ( bacille Calmette-Guérin ) that is closely related to the bacteria that cause TB.
- Have been treated with medicines, such as corticosteroids , that can affect your immune system .
- Are infected with the human immunodeficiency virus (HIV).
- Have a skin rash that may make it hard to read the skin test.
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?) .
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 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.
How It Feels
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.
After 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 fever.
- Swelling in your arm.
- Swollen lymph nodes in your armpit.
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:
- High-risk group includes people who have human immunodeficiency virus (HIV) , those who have had close recent contact with a person who has an active TB infection, and those who have symptoms or a chest X-ray that shows TB. Other people at high risk for tuberculosis include those who take medicines that contain corticosteroids for a long time or people taking tumor necrosis factor-alpha antagonists (used to treat rheumatoid arthritis and Crohn's disease ).
- Moderate-risk group 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 tuberculosis include people who are 10% or more below their ideal body weight and people who have kidney failure , diabetes , leukemia , cancer, or those who have had part of their stomach removed (gastrectomy).
- Low-risk group includes people who do not have any possible exposure to TB listed in the other risk groups.
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.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- A BCG (bacille Calmette-Guérin) vaccination. If you have had a BCG vaccination, you may have a positive PPD skin test.
- Taking medicines that suppress the immune system , such as corticosteroids.
- Conditions that weaken the immune system, such as HIV infection or cancer. The result also may be affected if a person is severely malnourished.
- Some vaccinations for infections, such as measles , mumps , rubella , polio , or chickenpox , given within 6 weeks before the tuberculin test. A recent infection with one of these viruses can also interfere with test results for a short period of time. The skin test also may be positive if the person has an infection caused by a mycobacterium other than the one that causes TB.
- A very recent TB infection. It takes 2 to 10 weeks for the immune system to react to TB bacteria.
- Age younger than 3 months old. A baby's immune system is not fully developed at this age.
- A "booster effect." This tends to occur in people who get
regular TB skin tests, such as health care workers. The booster effect is a
weak or no reaction to one TB skin test followed by a strong reaction from a
second test. But the strong reaction to the second test does not mean that the
person has just become infected with TB. Instead, the reaction may be because
- A TB infection that occurred long ago. Over time, a person's immune system tends to stop reacting strongly to the TB bacteria. So the first TB skin test stimulates (boosts) the immune system, which then reacts strongly to the second skin test.
- The BCG vaccine, which contains bacteria similar to the bacteria that cause TB. A person's immune system does not react as strongly to the vaccine bacteria, and this tends to diminish over time. So the person's immune system is boosted by the first skin test and reacts strongly to the second test.
- Infection with bacteria similar to Mycobacterium tuberculosis. This may also cause a first TB skin test to boost the immune system, allowing it to react strongly to a second test, even though a new TB infection has not occurred.
What To Think About
- The results of a tuberculin skin test alone cannot confirm an active TB infection. Other tests, such as a chest X-ray, sputum cytology, and sputum culture, may be done to confirm an active TB infection when a skin test is positive. A person who has a positive skin test or chest X-ray, but no TB symptoms, is usually thought to have a TB infection that cannot be passed to others (latent TB). For more information, see the topics Chest X-Ray, Sputum Cytology, and Sputum Culture.
- Among hospital workers or others who have periodic skin tests, a second test done within a few weeks of a negative test may be positive, even though the person was not infected between the two tests. These results (called the booster effect) may indicate a previous BCG vaccination or TB infection.
- About 5% of people who have inactive TB will develop active TB in 2 years. Another 5% of people with inactive TB will develop active TB in their lifetime. The chance of developing active TB is higher in children, older adults, and people with an impaired immune system .
- No more tests are needed for a person with a negative tuberculin skin test who has no symptoms of active infection and no history of being exposed to TB.
- Some people do not react to a tuberculin skin test even if they have tuberculosis. Conditions such as active TB, cancer, or autoimmune deficiency syndrome (AIDS) do not always respond normally to the TB antigens. In these cases, other skin tests may be done. If there is a skin reaction, then the tuberculin skin test is probably correct. But if there is no reaction, the person's immune system is likely to be too weak to respond normally to the tuberculin skin test.
- Rapid blood tests to diagnose TB have been approved by the U.S. Food and Drug Administration (FDA). These tests may be used instead of a tuberculin skin test. A rapid blood test may be able to tell if a person reacted to a skin test because of an active TB infection or a previous BCG vaccination. Rapid blood tests are also called interferon-gamma release assays (IGRAs).
- Rapid sputum tests that can detect TB bacteria in sputum have been approved by the FDA. These tests, called nucleic acid amplification tests (NAAs) can provide results within 24 hours. But they are done only when a person is strongly suspected of having TB.
- For more information about tuberculosis, see the topic Tuberculosis (TB).
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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