What is the Tuberculin Skin Test (TST)?
The
Tuberculin Skin Test (TST) is a diagnostic tool used to screen for latent or active
tuberculosis (TB) infection. It involves the intradermal injection of a small amount of purified protein derivative (PPD) into the forearm. The test is based on the delayed-type hypersensitivity reaction, and the results are read between 48 to 72 hours after administration.
How is the Reaction Measured?
After the injection, the test site is examined for induration (swelling and hardness) rather than redness. The diameter of induration is measured in millimeters. The size of the induration that is considered a positive result varies depending on the individual's risk factors and history of
BCG vaccination.
What are the Criteria for a Positive TST?
The interpretation of a
positive TST depends on several factors, including the person's risk of TB infection and progression to active TB disease. Generally, an induration of 5 mm or more is considered positive in individuals with HIV infection, recent contacts of TB cases, or those with chest radiographs consistent with prior TB. An induration of 10 mm or more is positive for recent immigrants from high-prevalence countries, injection drug users, and residents of high-risk settings. For those with no known risk factors, an induration of 15 mm or more is typically required to classify the test as positive.
What are the Limitations of the TST?
The TST has several
limitations. It cannot distinguish between latent and active TB infection, nor can it differentiate TB infection from a prior BCG vaccination or exposure to non-tuberculous mycobacteria. Additionally, false-positive and false-negative results can occur due to various factors, such as improper administration, interpretation, or the presence of immunosuppression.
How Does TST Compare to Other Diagnostic Tests?
In recent years,
Interferon Gamma Release Assays (IGRAs) have emerged as an alternative to the TST. IGRAs are blood tests that measure the immune response to specific TB antigens and do not cross-react with the BCG vaccine. They are particularly useful in BCG-vaccinated individuals and in situations where the TST may be unreliable. However, IGRAs are more costly and require laboratory facilities.
What is the Role of TST in Epidemiology?
The TST plays a crucial role in
epidemiological studies by helping to estimate the prevalence of latent TB infection in populations. It is often used in screening programs for high-risk groups, such as healthcare workers, prisoners, and individuals from regions with high TB incidence. The data obtained from TST screening can inform public health interventions and strategies to control and prevent TB.
What are the Considerations for TST Implementation?
Implementing TST requires careful consideration of the population being tested, the resources available, and the potential impact of testing on the community. It is important to ensure proper training for healthcare workers administering and interpreting the test. Additionally, follow-up care and treatment for those with positive results are essential to prevent the progression of latent TB infection to active disease. Conclusion
The
Tuberculin Skin Test remains a valuable tool in the fight against tuberculosis, especially for identifying latent infections. Despite its limitations, it provides critical data for understanding the epidemiology of TB and guiding public health policies. Ongoing research and technological advancements may improve TB diagnostics, but the TST continues to be an essential component of TB control efforts worldwide.
