The Tuberculin Skin Test (TST), also known as the Mantoux or Purified Protein Derivative (PPD) test, is a screening tool used to determine if a person has been infected with Mycobacterium tuberculosis bacteria. The test detects the body’s immune reaction to bacterial proteins, suggesting exposure has occurred at some point. The TST screens for infection, including latent TB infection (LTBI), but is not a diagnostic test for active tuberculosis (TB) disease. Identifying latent infection allows for preventive treatment, stopping the bacteria from developing into active, transmissible disease.
Understanding the Tuberculin Skin Test Injection
The initial “bubble” seen immediately after the injection is expected and its disappearance is normal. The TST is an intradermal injection, delivering a small amount of tuberculin solution just under the top layer of skin, usually on the inner forearm. This shallow injection creates a small, raised bump called a wheal or bleb, confirming the correct technique was used.
This temporary wheal is a physical reaction to the injected fluid. Within minutes to an hour, the body reabsorbs the solution, and the bubble flattens and disappears. The test measures a delayed hypersensitivity response that takes days to develop, not minutes. Therefore, the fading of the initial bump does not invalidate the test; it simply means the fluid has been absorbed.
The Critical Timing for TST Reading
The validity of the TST relies on a precise reading window because the biological reaction takes time to fully manifest. A trained healthcare professional must examine and measure the reaction between 48 and 72 hours after the tuberculin was administered. This 2-to-3-day period is when the delayed hypersensitivity reaction is expected to be at its peak, providing the most accurate indication of prior exposure.
Reading the test before 48 hours may result in a false-negative result because the immune response has not fully developed. Waiting past the 72-hour mark can also compromise the result, as the reaction may begin to fade, leading to an inaccurate reading. If the patient does not return within the required 72-hour window, the test is considered invalid and must be repeated.
How Test Results Are Interpreted
Interpreting the TST focuses on measuring induration, which is a firm, raised, hardened area at the injection site. Redness, or erythema, is disregarded during the measurement process, as it does not indicate a positive result. The healthcare provider uses a ruler to measure the diameter of the induration, specifically across the forearm, and records the measurement in millimeters. The threshold for what constitutes a positive result is not a single number but varies depending on the patient’s individual risk factors for TB infection.
Tiered Interpretation of Induration
The interpretation is tiered based on risk factors:
An induration of 5 millimeters or more is positive for people with the highest risk, such as those who are HIV-positive, recent close contacts of a person with active TB, or those who are significantly immunosuppressed.
An induration of 10 millimeters or more is positive for individuals with certain medical conditions, those who have recently arrived from countries with high TB prevalence, or people who work in high-risk settings like healthcare.
A reaction of 15 millimeters or more is considered positive for all people, even those with no known risk factors for TB exposure.
This tiered interpretation ensures that the result is accurately assessed within the context of the person’s medical history and potential for infection.
Necessary Follow-Up Actions
A negative TST result, typically an induration of 0 to 4 millimeters, generally means the person has not been infected with the TB bacteria. No further action is usually required after a negative result unless the person has had a very recent, known exposure to an infectious case. In cases of recent exposure, a repeat test may be recommended 8 to 12 weeks after the last contact, as it takes time for the immune system to develop a detectable reaction.
A positive TST result indicates that the person has been infected with Mycobacterium tuberculosis and requires further evaluation to distinguish between latent TB infection (LTBI) and active TB disease. The immediate next step is typically a chest X-ray to check for signs of active disease in the lungs. If the chest X-ray is abnormal or if the patient reports symptoms like persistent cough, fever, or unexplained weight loss, further diagnostic tests, such as collecting sputum samples, are needed. If the workup rules out active TB, the person is diagnosed with LTBI, which necessitates treatment to prevent the latent bacteria from developing into contagious, active TB disease in the future.