A Tuberculin Skin Test (TST), also known as a Mantoux test, is a method to determine if a person has been infected with Mycobacterium tuberculosis, the bacteria responsible for tuberculosis (TB). This test involves injecting a small amount of a substance called tuberculin, or purified protein derivative (PPD), just under the skin, typically on the inner forearm. The purpose of the TST is to detect a person’s immune response to these bacterial proteins, indicating prior exposure to the TB bacteria. For accurate results, a healthcare professional must observe the injection site within 48 to 72 hours following the injection, with the 48-hour mark being a key time for evaluation.
Recognizing a Positive Reaction
A positive TB skin test reaction is identified by induration, a firm, raised, hardened area at the injection site. This induration feels like a distinct bump under the skin when gently touched. The size of this hardened area, not any accompanying redness or bruising, indicates a positive result. Redness, itching, or bruising at the site are not indicators of a positive reaction and should not be measured.
Reading the Test Site
Interpreting a TB skin test involves measuring the diameter of the induration across the forearm, perpendicular to the long axis of the arm. This measurement is crucial, as different sizes of induration are considered positive based on an individual’s risk factors for TB infection.
An induration of 5 millimeters (mm) or more is positive for individuals with weakened immune systems (e.g., HIV, organ transplant recipients) or recent close contact with someone with active TB.
A reaction of 10 mm or more is positive for people who have recently immigrated from countries with high TB prevalence, healthcare workers, or those living in high-risk congregate settings.
An induration of 15 mm or more is positive for individuals with no known risk factors.
If no induration is present, the result is considered negative.
Meaning of a Positive Result
A positive TST result indicates infection with Mycobacterium tuberculosis bacteria. However, it is important to understand that a positive test does not automatically mean the person has active tuberculosis disease.
There are two main states of TB infection: latent TB infection (LTBI) and active TB disease. In latent TB infection, the bacteria are present but inactive, causing no symptoms and not being contagious. While individuals with LTBI do not feel sick and cannot spread the bacteria, the infection can progress to active TB disease in the future. Active TB disease means the bacteria are multiplying and causing symptoms, making the person contagious. Symptoms of active TB can include a persistent cough, chest pain, fever, weight loss, and night sweats.
Follow-Up Actions
Upon receiving a positive TB skin test result, consulting a healthcare professional is the next essential step for proper interpretation and further evaluation. A positive TST indicates the presence of TB bacteria, but additional tests are needed to differentiate between latent TB infection and active TB disease.
A healthcare provider will typically order a chest X-ray to check for signs of active infection in the lungs. Sputum tests, which analyze mucus coughed up from the lungs, may also be conducted to identify active TB bacteria. These follow-up evaluations help determine the appropriate course of action, which may include treatment for latent TB infection to prevent active disease, or immediate treatment for active TB.