Mantoux Test
The Mantoux test, also known as the tuberculin skin test (TST), is used to evaluate for the presence of a latent tuberculosis (TB) infection or to assess the immune response to Mycobacterium tuberculosis, the bacterium that causes TB. Here’s how the Mantoux test is used:
- Screening for Latent Tuberculosis Infection (LTBI): The Mantoux test is commonly used as a screening tool to identify individuals who have been exposed to TB bacteria and have developed a latent TB infection. During the test, a small amount of purified protein derivative (PPD), which contains antigens derived from M. tuberculosis, is injected under the top layer of the skin (intradermally). The injection site is typically on the forearm.
- Immune Response Assessment: After the PPD injection, the body's immune system reacts to the TB antigens present in the PPD. If an individual has been previously exposed to TB bacteria, their immune system will recognize the antigens and mount an immune response, leading to the formation of a raised, reddened area at the injection site. The size of this reaction is measured 48 to 72 hours after the injection to assess the individual's immune response.
- Interpretation of Results: The size of the reaction (induration) is measured in millimeters using a ruler. A positive reaction is determined based on the size of the induration, taking into account the individual's risk factors for TB exposure. The interpretation of the Mantoux test results depends on guidelines established by health authorities. Generally, an induration of 5 mm or more is considered positive in certain high-risk groups (such as close contacts of individuals with active TB, individuals with HIV infection, and individuals with immunosuppressive conditions), while a larger induration (e.g., 10 mm or more) may be considered positive in other populations.
- Follow-up Testing: In some cases, further testing may be necessary to confirm the diagnosis or rule out active TB disease. Individuals with positive Mantoux test results may undergo additional testing, such as chest X-rays, sputum tests, or interferon-gamma release assays (IGRAs), to determine whether they have latent TB infection or active TB disease.