A positive purified protein derivative (PPD) test indicates that you have been infected with the bacteria that cause tuberculosis (TB). However, a positive PPD test does not mean you have active TB disease. Further testing is always necessary to determine if the infection is active.
Understanding the PPD Test
The PPD test, also known as the Mantoux tuberculin skin test, is a common method used to screen for TB infection. A small amount of fluid containing TB antigens is injected under the skin. The presence of a raised, hardened area (induration) at the injection site after 48-72 hours indicates a positive result. The size of the induration, along with other factors such as risk factors for TB, determines whether the result is considered positive.
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What a Positive Result Means: A positive PPD test reveals exposure to the Mycobacterium tuberculosis bacteria, meaning the bacteria are present in your body. This is called latent TB infection. You are not contagious and do not show symptoms. However, there’s a risk the infection could become active TB later.
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What a Positive Result Doesn't Mean: A positive PPD test does not automatically mean you have active TB disease. Active TB is contagious and causes symptoms like coughing, fever, and fatigue. Additional tests like a chest X-ray and/or blood tests (like Interferon-Gamma Release Assays or IGRAs) are needed to determine if the infection has progressed to active TB.
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Interpreting the Size of Induration: The size of the induration that defines a positive result varies depending on your risk factors for TB:
- ≥5 mm: Individuals with HIV, recent contacts of people with infectious TB, those with organ transplants, or people with other conditions that weaken the immune system.
- ≥10 mm: Individuals with increased risk factors for TB based on immigration history, certain medical conditions, or other risk factors.
- ≥15 mm: Individuals with a low risk for TB.
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Next Steps After a Positive Result: If your PPD test is positive, your healthcare provider will order further tests to determine if you have active TB. If latent TB is confirmed, you'll likely be offered treatment to prevent the infection from becoming active.
References and Sources:
The information above is supported by numerous credible sources including the CDC, reputable hospitals, and health departments. These sources consistently emphasize that a positive PPD test only indicates past exposure to the TB bacteria and necessitates further testing to determine the presence of active disease. Further details can be found at sources like the CDC's website on tuberculosis testing (https://www.cdc.gov/tb/hcp/testing-diagnosis/tuberculin-skin-test.html).
