What is the Tuberculin Skin Test?
The
Tuberculin Skin Test (TST), also known as the Mantoux test, is a diagnostic tool used to determine if an individual has been exposed to
Mycobacterium tuberculosis, the bacterium responsible for tuberculosis (TB). The test involves the intradermal injection of a purified protein derivative (PPD) of the tuberculin antigen. A subsequent skin reaction is measured to assess whether the person has been infected with the TB bacteria.
How is the Tuberculin Skin Test Administered?
The TST is administered by injecting a small amount of tuberculin PPD into the inner surface of the forearm. The injection should produce a small, pale elevation of the skin (a wheal) which is approximately 6-10 mm in diameter. The test site is then evaluated after 48 to 72 hours to measure the induration (swelling) rather than the redness. What Does a Positive Result Indicate?
A positive TST result typically indicates that a person has been infected with the TB bacteria. However, it does not differentiate between
latent TB infection and active TB disease. A positive result means that further
diagnostic testing is required, such as a chest X-ray or a sputum test, to determine the presence of active TB disease.
Can a Tuberculin Skin Test Have False Results?
Yes, the TST can yield false-positive or false-negative results. A false-positive result may occur if the person has been vaccinated with the
BCG vaccine or exposed to non-tuberculous mycobacteria. False-negative results can occur in individuals with weakened immune systems, recent TB infection, or in very young children. Therefore, results should always be interpreted in the context of the individual's risk factors and clinical presentation.
What Are the Limitations of the Tuberculin Skin Test?
The TST has several limitations. It requires a return visit to measure the reaction, which can be inconvenient. Additionally, it can have cross-reactivity with the BCG vaccine and non-tuberculous mycobacteria. It is also less reliable in populations with high rates of BCG vaccination. In some cases, an
Interferon Gamma Release Assay (IGRA) might be used as an alternative, especially in BCG-vaccinated individuals.
Who Should Be Tested with the Tuberculin Skin Test?
The TST is recommended for individuals who are at high risk of TB infection. This includes people who have been in close contact with someone with active TB, healthcare workers, individuals with HIV infection, and those with certain medical conditions that increase the risk of TB. It is also used in routine screening programs in high-risk settings.
How is the Reaction to the Tuberculin Skin Test Measured?
The reaction to the TST is measured in millimeters of induration at the site of the test. The interpretation of the size of the induration varies based on the individual's risk factors. For example, an induration of 5mm or more is considered positive in HIV-infected persons, while 10mm or more is positive in recent immigrants from high-prevalence countries. For low-risk individuals, 15mm or more is considered a positive result.
Are There Any Side Effects of the Tuberculin Skin Test?
The TST is generally safe, but some individuals may experience minor side effects such as mild itching, redness, or swelling at the injection site. Severe allergic reactions are rare. If any unusual symptoms occur, it is important to contact a healthcare provider. Conclusion
The Tuberculin Skin Test remains an important tool in the
diagnosis of tuberculosis, particularly in resource-limited settings. Despite its limitations, it is valuable for identifying individuals who may have been exposed to TB and require further evaluation or treatment. Understanding the nuances of test administration, interpretation, and potential pitfalls is essential for healthcare providers managing TB care.
