The Mantoux tuberculin skin test, also known as the PPD test, serves as a common screening method for tuberculosis (TB) infection. This test aims to determine if an individual has been exposed to the bacteria that cause TB. It involves injecting a small amount of purified protein derivative (PPD) into the forearm.
Identifying a Positive Reaction
A positive TB skin test reaction is identified by a firm, raised bump at the injection site, known as induration. This palpable, hardened area’s size, not redness, determines a positive result. While the area might appear red or swollen, redness alone does not indicate a positive test.
Healthcare professionals measure the diameter of this induration in millimeters, perpendicular to the long axis of the forearm. This measurement is taken between 48 and 72 hours after the injection.
The specific size threshold for a positive result varies depending on an individual’s risk factors for TB exposure. For instance, an induration of 5 millimeters or more is considered positive for individuals with HIV, recent close contact with someone with TB, or those who are immunosuppressed. For people born in countries where TB is common, healthcare workers, or those living in high-risk congregate settings like nursing homes, a 10-millimeter induration or greater is considered positive. For individuals with no known risk factors for TB, an induration of 15 millimeters or more indicates a positive result.
In contrast, a negative reaction shows no induration, or only a very small bump that disappears quickly, possibly with just a small red mark from the injection itself. It is important that only a trained healthcare professional interprets the results, as they consider individual risk factors alongside the measurement.
Understanding Your Results
A positive TB skin test indicates that a person has been exposed to TB bacteria or has a latent TB infection. However, it does not automatically mean the individual has active TB disease. The body’s immune system can contain the bacteria, preventing them from causing illness.
Further diagnostic tests are necessary to determine if active TB disease is present. These tests may include a chest X-ray to look for signs of infection in the lungs or sputum tests, which analyze mucus coughed up from the lungs.
Latent TB infection means that the TB bacteria are present in the body but are inactive, causing no symptoms, and the person is not contagious. Active TB disease, conversely, means the bacteria are multiplying, symptoms are present (such as persistent cough, fever, or night sweats), and the person may be able to spread the bacteria to others.
Following a positive TB skin test, it is important to consult a healthcare provider for proper diagnosis and counseling. They will discuss the implications of the positive result and determine if further evaluation or treatment for latent TB is appropriate to prevent the development of active disease.