In tuberculin test, the tuberculin antigen is injected in the skin of your forearm. If you have been exposed to tuberculosis (TB)—an infectious disease caused by a microorganism called Mycobacterium tuberculosis—your body’s immune response will cause an inflammatory reaction at the injection site. However, the tuberculin test cannot distinguish between active and dormant tuberculosis infection, and the results are not definitive. In adults, tuberculin test is most often performed in people who have had a chest x-ray that suggests possible tuberculosis, or in those with a suspected recent exposure.
Purpose of the Tuberculin Test
- To screen a person for TB exposure (This test may be performed yearly in people who have a weakened immune system.)
- To detect previous infection with the bacterium M. tuberculosis and to determine the need for further testing
- To distinguish tuberculosis from other infections that affect the lungs
Who Performs Tuberculin Test
- A nurse or a doctor
Special Concerns about Tuberculin Test
- Multipuncture tuberculin tests, such as the tine test, are most often used for screening purposes in apparently healthy individuals. However, a positive reaction with these less accurate tests generally requires tuberculin injection for confirmation.
- This test is not generally done after a diagnosis of tuberculosis has been made, or in individuals known to have had a positive skin test reaction in the past.
- People whose immune system is compromised due to old age, poor nutrition, or chronic illness; those who were recently vaccinated for measles, rubella, mumps, or another infectious disease; and those receiving treatment with steroid medications may not react to this test despite exposure to tuberculosis; this is known as a false-negative result.
- False-negative results are also possible if tuberculosis exposure occurred less than 10 weeks before the test.
Before the Tuberculin Test
- Your doctor or another health care provider will interview you regarding your medical history before the test. You should mention previous active tuberculosis and the results of prior skin tests.
- Tell your doctor if you have a medical condition or take certain drugs that can affect the immune system, such as steroids, which can cause inaccurate test results.
What You Experience During Tuberculin Test
- You sit with your arm extended on a table or some other flat surface.
- The person performing the test will clean your upper forearm with alcohol, let it dry, and administer the tuberculin injection. The injection may cause brief discomfort.
- The site is often circled with indelible ink for easy identification.
- You must return to the testing site in 2 to 3 days, since skin reactions to the tuberculin antigen typically develop within this period. You may experience some itching during the interim, but do not scratch the area.
Risks and Complications of Tuberculin Test
- Rarely, this test may cause an acute allergic reaction.
- In people with an active tuberculosis infection or those who have previously been vaccinated against the disease, reaction to the skin test may be severe, causing skin breakdown or ulceration.
- In people with a previous positive PPD test, there is a small risk of severe redness and swelling at the site of the test.
- Tuberculin test cases a brief sting as a needle is inserted just below the skin surface.
After the Tuberculin Test
- You may leave the testing facility immediately after the injection has been administered.
- A follow-up appointment will be scheduled for 2 to 3 days after the injection.
- Call your doctor if a severe skin reaction occurs.
Results of Tuberculin Test
- A positive reaction to the tuberculin test will appear as a red, raised area at the injection site. Redness alone without the raised area, however, does not constitute a positive test.
- If you do have a positive skin reaction, the cause is not always tuberculosis—it may result, for example, from infection with another, related bacterium. For this reason, additional tests, such as a sputum culture and chest x-rays, will be necessary to confirm the diagnosis.
- If the reaction is borderline, the skin test may be repeated.
- If the skin reaction is negative, but you have other signs suggesting possible tuberculosis, additional tests may be performed.
The Johns Hopkins Consumer Guide to Medical Tests
Simeon Margolis, M.D., Ph.D., Medical Editor
Updated by Remedy Health Media