what is tst test

3 min read 14-03-2025

Tuberculin skin testing (TST), also known as the Mantoux tuberculin skin test, is a simple, inexpensive diagnostic test used to determine if a person has been infected with Mycobacterium tuberculosis, the bacteria that cause tuberculosis (TB). It doesn't directly detect active TB disease, but rather indicates past exposure to the bacteria. This is crucial because latent TB infection (LTBI) can remain dormant for years before potentially progressing to active TB.

How Does a TST Test Work?

The TST involves injecting a small amount of tuberculin, a purified protein derivative (PPD) from M. tuberculosis, just under the skin of the forearm. This injection creates a small wheal (raised bump). After 48-72 hours, a healthcare professional will measure the size of the induration (hardened area) at the injection site. The size of the induration is the key to interpreting the results.

Interpreting TST Results:

The interpretation of a TST is not straightforward and depends on several factors, including:

The size of the induration: A larger induration generally indicates a higher likelihood of infection. The size thresholds for a positive test vary depending on individual risk factors (like immunocompromised status or recent close contact with a person with active TB). A healthcare provider will consider these factors when interpreting results.
Risk factors: Individuals with certain risk factors (e.g., recent immigration from high-TB-prevalence countries, working in healthcare settings, or having weakened immune systems) may be considered to have a positive TST with a smaller induration size.
Previous BCG vaccination: Bacille Calmette-Guérin (BCG) is a vaccine used in many parts of the world to prevent TB. It can cause a positive TST result, even in the absence of TB infection. This needs to be taken into account.

What a Positive TST Result Means: A positive TST result suggests that you have been exposed to M. tuberculosis and have developed an immune response. This doesn't necessarily mean you have active TB disease. Further testing is needed to confirm whether you have LTBI or active TB. This usually involves a chest X-ray and possibly other tests.

What a Negative TST Result Means: A negative TST result means that the test did not detect evidence of exposure to M. tuberculosis. However, it's still possible to have a very early infection or a very weak immune response that the TST may not detect.

Who Should Get a TST Test?

The CDC and other public health organizations recommend TST for specific groups at higher risk of TB infection, including:

Individuals with close contact with someone diagnosed with TB: Family members, close friends, and co-workers.
Immigrants and refugees from high-TB-prevalence countries: These areas have significantly higher rates of TB infection.
Healthcare workers: They are in frequent contact with individuals who may have TB.
People with weakened immune systems: Individuals with HIV/AIDS, organ transplant recipients, and those receiving immunosuppressive medications are more vulnerable.
Residents and employees of long-term care facilities: These environments can facilitate the spread of TB.
Injection drug users: This high-risk group is more susceptible to TB infection.

Limitations of the TST Test

It's important to understand that the TST has limitations:

False positives: Factors like BCG vaccination or certain non-tuberculosis mycobacterial infections can lead to false positive results.
False negatives: Individuals with weakened immune systems may not mount a detectable immune response, leading to a false negative. Very early infections may also yield a false negative result.
Does not distinguish between latent and active TB: Further testing is always required to determine the presence or absence of active TB disease.

Alternative to TST: Interferon-Gamma Release Assays (IGRAs)

Interferon-Gamma Release Assays (IGRAs) are blood tests that can also detect TB infection. IGRAs are less affected by BCG vaccination compared to TST and are becoming increasingly common as an alternative testing method. Your doctor can help you decide which test is best for you.

Conclusion

The TST remains a valuable tool in the detection of past exposure to M. tuberculosis. However, it's crucial to remember that a positive result doesn't automatically mean active TB disease. Further evaluation is always necessary to determine the appropriate course of action. If you have concerns about TB infection, talk to your healthcare provider. They can assess your risk factors and order the appropriate diagnostic tests.