A 2-step TB skin test (TST), also known as the Mantoux tuberculin skin test, is a method used to screen adults who will be periodically retested for Mycobacterium tuberculosis infection, the bacteria that causes tuberculosis (TB). This approach helps distinguish between a new infection and a boosted reaction from a previous TB exposure.
Purpose of a 2-Step TST
The primary purpose of a 2-step TST is to reduce the likelihood of misinterpreting a boosted reaction to a previously acquired TB infection as a new infection in individuals who undergo regular TB testing. This is particularly important for healthcare workers and others at increased risk of TB exposure.
How the 2-Step TST Works
The 2-step TST involves two separate TST administrations:
- First TST: A TST is administered and read 48-72 hours later to measure the induration (swelling) in millimeters. If the result is negative, a second TST is administered.
- Second TST: The second TST is administered 1-3 weeks after the first negative test. This second test aims to detect a "boosted" immune response. A boosted reaction occurs when the initial TST stimulates a dormant immune response from a previous TB infection that had waned over time.
Interpreting the Results
- Positive First TST: Indicates a likely TB infection (latent or active). Further evaluation, such as a chest X-ray and additional tests, is required.
- Negative First TST, Positive Second TST: Indicates a "boosted" reaction from a previous TB infection. This is interpreted as a positive result, indicating prior exposure to TB.
- Negative First TST, Negative Second TST: Indicates a negative result. The individual is presumed not to be infected with TB at the time of testing.
Why is a 2-Step TST Important?
Without the 2-step approach, a healthcare worker, for example, who had a TB infection years ago might have a negative initial TST. If they were then exposed to TB and had a TST repeated later, the initial exposure could "boost" the reaction, making it appear as though they were newly infected. The 2-step TST helps identify the baseline response and avoid this misinterpretation.
Recommendations for Use
The 2-step TST is recommended for initial TB screening of adults who will undergo periodic repeat testing, such as:
- Healthcare workers
- Long-term care facility residents
- Correctional facility staff and inmates
- Individuals starting immunosuppressive therapy
