What is Tuberculosis (TB) Skin Test?
Tuberculosis (TB) is a highly contagious disease. It’s caused by an infection of bacteria called Mycobacterium tuberculosis (Mtb).
The tuberculosis skin test is also known as the tuberculin test or PPD test.
The PPD test is used to determine if someone has developed an immune response to the bacterium that causes tuberculosis (TB).
Why is Tuberculosis (TB) Skin Test performed?
This test may be performed if you have been exposed to a person with TB or if you have a clinical condition or risk factor that makes progression to active TB more likely
What should I know before receiving Tuberculosis (TB) Skin Test?
The test typically does not produce side effects. There is a very slight risk of having a severe reaction to the test, including swelling and redness of the arm, particularly in people who have had tuberculosis or been infected previously and in those who have previously had the BCG vaccine. Allergic reactions are also rare complications.
Live bacteria are not used in the test, so there is no chance of developing tuberculosis from the test.
How to prepare for Tuberculosis (TB) Skin Test?
This test doesn’t require any specific preparation. If you have any concern, talk to your doctor.
What happens during Tuberculosis (TB) Skin Test?
A TB skin test is done in two parts:
During one visit to a doctor’s office or clinic, a tiny amount of tuberculin is injected under the skin, usually in the forearm. Tuberculin is a sterile extract purified protein derivative (PPD) made from the bacteria that cause TB.
After receiving the injection, a small, pale bump will form at the site.
The second phase of the test takes place 48 to 72 hours later. At that time, your doctor will look at your skin to see how it reacted to the tuberculin. Your skin’s reaction will help your doctor determine if you’ve been infected with TB.
If you wait longer than 72 hours, you’ll have to start over with a new test and new injection.
If this is your first TB skin test and it’s negative, you may be asked to return in one to three weeks for a repeat test to ensure the results are the same.
What happens after Tuberculosis (TB) Skin Test?
If you have a positive test result and you have symptoms or are considered at high risk of TB exposure, you’ll likely be prescribed medications to clear up the infection and relieve symptoms.
If you’re low risk and have a positive test, your doctor may recommend a TB blood test to confirm the diagnosis. The TB skin test is less accurate than the blood test, so you could have a positive skin test and a negative blood test.
If you have any questions about the Tuberculosis (TB) Skin Test, please consult with your doctor to better understand your instructions.
Explanation of results
What do my results mean?
A healthcare practitioner will interpret a tuberculin skin test result by looking at the injection site on the person’s forearm at 48 or 72 hours (in most cases). A positive result will form a red and swollen raised circle at the site of the injection. The size (diameter) of the swollen raised circle determines whether exposure to TB has occurred. The size that is considered positive varies with the health status and age of the individual. Even when infected, children, the elderly, and people who are severely immune compromised (such as those with AIDS) may have smaller, delayed, or even negative reactions to the TST.
A positive TST or IGRA test result means that the person is likely to have been exposed to TB and the person may have a latent or active TB infection. If a healthcare practitioner suspects that someone has active tuberculosis, a history and physical examination and other tests, such as chest X-rays and AFB laboratory testing, are used to confirm the diagnosis.
A negative result means that it is likely that the person tested does not have a TB infection. However, it does not entirely rule out tuberculosis. It may mean that the person’s immune system has not responded to the antigen in the test or that it is too early to detect exposure. It takes about 6 weeks after infection before a person demonstrates a positive reaction to TB screening tests. If suspicion of TB remains high and a healthcare practitioner wants to confirm a negative or indeterminate result, the practitioner may repeat the TST or do an IGRA as an alternate follow-up test.
Occasionally, a person infected with or exposed to other Mycobacterium species, for example Mycobacterium kansasii, will give a false-positive TST result for TB. Positive results must be followed up by other tests such as chest X-rays to look for signs of active TB disease. If active TB disease is suspected, AFB testing including smears and cultures and sensitivity testing, may be used to confirm the diagnosis and determine the drug susceptibility for the M. tuberculosis infecting the person.
Depending on the laboratory and hospital, the normal range for Tuberculosis (TB) Skin Test may vary. Please discuss with your doctor any questions you may have about your test results.
Hello Health Group does not provide medical advice, diagnosis or treatment.
TB Skin Test. https://labtestsonline.org/tests/tb-skin-test. Accessed October 26, 2018.
How to Identify a Positive Tuberculosis (TB) Skin Test. https://www.healthline.com/health/what-does-a-positive-tb-test-look-like#skin-test-procedure. Accessed October 26, 2018.
Tuberculosis Skin Test (PPD Skin Test). https://www.medicinenet.com/tuberculosis_skin_test_ppd_skin_test/article.htm#tuberculosis_skin_test_facts. Accessed October 26, 2018.
Review Date: November 5, 2018 | Last Modified: September 12, 2019