Tuberculosis (TB) is a global public health concern. Early identification of infection is important for controlling its spread. TB tests are screening tools designed to detect the body’s immune response to Mycobacterium tuberculosis, the bacteria that cause TB. Understanding these test results is key to managing potential TB exposure and preventing active disease.
Appearance of a Positive Skin Test
A positive TB skin test, specifically the Mantoux tuberculin skin test (TST), presents as a firm, raised, hardened area, known as induration, at the injection site. This induration is the primary indicator of a reaction, not merely redness. Redness alone does not signify a positive result. A trained healthcare professional must visually inspect and gently palpate the site to accurately determine the presence and size of this hardened area.
The test is administered by injecting a small amount of tuberculin purified protein derivative (PPD) just under the skin on the inner forearm. After 48 to 72 hours, the site is examined for a reaction. The healthcare worker measures the induration’s diameter, as its size, not surrounding redness, guides the interpretation.
Meaning of a Positive Result
A positive TB skin test indicates that an individual has been infected with Mycobacterium tuberculosis bacteria. However, this does not automatically mean they have active TB disease. Instead, it typically signifies a latent TB infection (LTBI), where the bacteria are present in the body but remain inactive and do not cause symptoms. Individuals with LTBI usually do not feel sick and cannot transmit the bacteria to others.
A positive skin test can also occur in individuals who have received the Bacille Calmette-Guérin (BCG) vaccine, common in countries where TB is widespread. While BCG vaccination can cause a false-positive skin test, a positive result still warrants further evaluation for actual TB infection, as the skin test cannot differentiate between a reaction due to the vaccine and one due to true infection. The interpretation of the induration size is determined by a healthcare provider based on a person’s individual risk factors for TB exposure or progression to disease.
Next Steps After a Positive Test
Following a positive TB skin test result, consulting a healthcare professional for further evaluation is an important next step. The initial positive result suggests the presence of TB bacteria in the body, prompting additional diagnostic procedures to determine if the infection is latent or active. These follow-up tests help to establish a definitive diagnosis and guide appropriate management.
A chest X-ray is commonly performed to check for any signs of active TB disease in the lungs. While a chest X-ray can suggest TB, it cannot confirm the disease on its own, as other conditions may present similar findings. If active TB is suspected, sputum tests are conducted to look for the presence of TB bacteria. These tests involve analyzing mucus coughed up from the lungs to identify the bacteria and determine if the person is contagious.
Blood tests, specifically Interferon-Gamma Release Assays (IGRAs), may also be used as an alternative or confirmatory test. IGRAs are not affected by prior BCG vaccination, offering an advantage over the skin test in vaccinated individuals. Based on the results of these comprehensive evaluations, a healthcare provider will determine if the individual has latent TB infection or active TB disease and recommend suitable treatment or monitoring.