The Tuberculin Skin Test (TST), widely recognized as the Mantoux test or PPD test, is a common method for screening individuals for potential tuberculosis (TB) infection. This diagnostic procedure involves a small injection of a testing substance just beneath the skin’s surface. The TST helps healthcare providers determine if a person’s immune system has reacted to the bacteria that cause tuberculosis.
Understanding the TB Skin Test
The Tuberculin Skin Test detects a delayed-type hypersensitivity reaction to specific TB antigens. A healthcare professional injects a small amount of purified protein derivative (PPD) into the top layer of skin on the inner forearm. This injection is administered intradermally, into the dermis, the layer just below the epidermis.
A visible “bubble” or “bleb” is an expected and immediate outcome of an intradermal injection. Its appearance confirms that the PPD solution was delivered at the appropriate depth within the skin.
The Normal Disappearance of the Bleb
It is normal for the initial bleb or bubble that forms immediately after a TB skin test injection to disappear shortly after. This rapid disappearance does not indicate that the test was incorrectly administered or that it will not yield accurate results. The fluid from the bleb is quickly absorbed by the body’s tissues, leaving no immediate visible mark at the injection site.
The immune reaction to the PPD antigens develops later, between 48 and 72 hours after the injection. This delayed reaction is distinct from the immediate, temporary bleb. The test relies on this delayed response, not on the transient presence of the initial fluid bubble.
Interpreting Your TB Test Results
When the TB skin test is read 48 to 72 hours post-injection, a trained healthcare professional assesses the site. The evaluation focuses on measuring the diameter of any induration, which is a raised, hardened area of skin. Redness (erythema) alone, without a palpable induration, is not considered a positive result or measured.
The interpretation of a positive or negative result depends on the measured size of the induration in millimeters, alongside the individual’s specific risk factors for TB infection. An induration of 5 millimeters or more may be considered positive in individuals with weakened immune systems, recent contact with a TB case, or certain medical conditions. An induration of 10 millimeters or more can be positive for recent immigrants from high-prevalence countries or healthcare workers. For individuals with no known risk factors, an induration of 15 millimeters or more is considered positive.
Important Considerations After Testing
After receiving a TB skin test, avoid scratching, rubbing, or covering the injection site with bandages or tape, as this could affect the test’s accuracy or cause irritation. You can gently wash the area with water, but avoid scrubbing. If the site becomes itchy, a cold cloth or ice cube can be applied for relief.
Returning to your healthcare provider for the official reading within the specified 48 to 72-hour window is necessary. Only a trained healthcare professional can accurately interpret the results by properly measuring any induration. If the test result is positive, it indicates the presence of TB bacteria in the body, and further evaluation, such as a chest X-ray or blood test, may be required to determine if it is latent or active TB disease. A negative result suggests no TB infection, and no further action is needed unless there is an ongoing risk of exposure.