What is the Tuberculin Skin Test (TST)?
The
Tuberculin Skin Test (TST), also known as the Mantoux test, is a diagnostic tool used to determine whether a person has been infected with
Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB). It involves the intradermal injection of purified protein derivative (PPD) into the forearm and observing the skin reaction 48 to 72 hours later.
How Does the TST Work?
The test works by injecting a small amount of PPD into the skin. If a person has been exposed to the TB bacteria, their immune system will recognize the PPD as a foreign substance and react to it, causing a raised, hard area or swelling at the injection site. This reaction is measured in millimeters and is interpreted based on the individual's risk factors.Who Should Get Tested?
The TST is recommended for individuals at high risk of TB infection, such as those who have been in close contact with someone with active TB, healthcare workers, individuals with weakened immune systems, and those who have lived or traveled in regions with high TB prevalence. It is also used for screening in certain occupational settings and populations where TB exposure is more likely.How is the TST Interpreted?
The interpretation of the TST depends on the size of the induration and the person's risk factors. A reaction of 5 mm or more is considered positive for individuals with HIV, recent contacts of infectious TB cases, or those with chest X-ray findings suggestive of previous TB. For healthcare workers and individuals with other high-risk conditions, a reaction of 10 mm or more is considered positive. In low-risk individuals, a reaction of 15 mm or more is typically considered positive.What are the Limitations of the TST?
The TST has several limitations. False-positive results can occur in individuals who have received the
BCG vaccine or have been exposed to non-tuberculous mycobacteria. False-negative results may occur in individuals with weakened immune systems, such as those with HIV, or in those who have been recently infected, as it may take several weeks for the immune system to react to the PPD. Therefore, the TST must be interpreted in the context of the individual's history and risk factors.
What are the Alternatives to the TST?
An alternative to the TST is the
Interferon Gamma Release Assays (IGRAs), which are blood tests that measure immune response to TB antigens. IGRAs are more specific than the TST, particularly in individuals who have received the BCG vaccine. They are also useful in situations where frequent testing is required, as there is no booster effect with repeated tests.
What is the Role of the TST in Infectious Disease Control?
The TST plays a critical role in the control of TB as it helps identify individuals who have been infected and may benefit from preventive treatment. This is particularly important in settings with high-risk populations, as early detection and treatment of latent TB infection can prevent the progression to active TB disease, thereby reducing transmission.What Should Be Done After a Positive TST?
A positive TST indicates that a person has been infected with TB bacteria, but it does not confirm active TB disease. Further evaluation, including a medical history, physical examination, chest X-ray, and possibly sputum tests, is necessary to determine if the person has active TB disease or latent TB infection. Treatment decisions are based on these findings and the individual's risk factors.Conclusion
The Tuberculin Skin Test is a valuable diagnostic tool in the identification of TB infection. Despite its limitations, it remains widely used, especially in resource-limited settings. Proper interpretation and follow-up are crucial to ensure accurate diagnosis and effective disease management. For more precise detection, particularly in vaccinated populations or those with frequent test requirements,
IGRAs offer a viable alternative.
