The tuberculin skin test, commonly known as the PPD test, serves as a screening tool to identify individuals who may have been exposed to Mycobacterium tuberculosis, the bacteria responsible for tuberculosis. This test helps determine if a person’s immune system has reacted to TB proteins, indicating a possible infection. Its primary purpose is to detect a past or current TB infection, rather than diagnosing active disease.
Administering the TB Skin Test
The TB skin test is administered on the inner surface of the forearm, about 2 to 4 inches below the elbow, in an area free of scars, sores, or excessive hair. A healthcare professional cleans the site with an alcohol swab and allows it to dry completely before the injection.
A small needle, usually a 27-gauge, with the bevel facing upward, is used. The needle is inserted slowly into the top layer of the skin at a shallow angle, between 5 to 15 degrees. This creates a small, pale, raised bump, often called a wheal, typically 6 to 10 millimeters in diameter. This wheal indicates proper intradermal injection and usually disappears within a few hours.
Reading and Understanding the Results
The results of a TB skin test are read by a healthcare provider between 48 and 72 hours after the injection. This timeframe is important because reading the test too early or too late can lead to inaccurate interpretations. The healthcare provider examines the injection site under good light, focusing on feeling for a firm, raised area, known as induration, rather than just looking at redness.
A negative reaction is no induration, or only a very small bump, less than 5 millimeters in diameter. Redness alone, without palpable swelling, does not indicate a positive result. A negative test suggests that the person has likely not been infected with TB bacteria.
A positive reaction shows a firm, raised area of swelling at the injection site. The induration is carefully measured in millimeters across the forearm. The interpretation of a positive result depends on the size of this induration and individual risk factors.
For example, an induration of 5 millimeters or more can be considered positive for individuals with weakened immune systems, recent contact with someone with TB, or those with chest X-ray findings suggesting past TB. An induration of 10 millimeters or more is considered positive for recent immigrants from high-prevalence areas, residents or employees in high-risk settings, or children under four. For individuals with no known risk factors, an induration of 15 millimeters or more is considered a positive result. Blistering at the site, even without significant swelling, also indicates a positive reaction.
Next Steps After a Positive Result
A positive TB skin test indicates that a person has been infected with TB bacteria at some point. It does not differentiate between a latent TB infection, where the bacteria are present but inactive, and active TB disease, where the bacteria are actively causing illness.
Following a positive skin test, further evaluation is necessary to determine the next steps. This includes a chest X-ray to check for signs of active disease in the lungs. Blood tests, such as an Interferon-Gamma Release Assay (IGRA), may also be conducted. These additional tests help healthcare providers determine if the infection is latent or active, guiding appropriate management and potential treatment to prevent the development or spread of active tuberculosis.