A two-step TB test, also known as a two-step tuberculin skin test (TST), is a method used to screen for tuberculosis (TB) infection. It is used to help determine if a person has been infected with TB in the past and now has a diminished skin test reactivity. This procedure helps reduce the chance of a boosted reaction from an old TB infection being misinterpreted as a new infection.
Here's how it works:
- Baseline PPD test: The first step involves administering a standard tuberculin skin test (TST) using purified protein derivative (PPD).
- Second test: If the first test is negative, a second TST is administered one to three weeks later. This helps distinguish between a boosted reaction from a previous infection and a new infection.
Why use a two-step TB test?
- Reduced chance of false positives: It helps minimize the likelihood of misinterpreting a boosted reaction (a reaction to an old TB infection) as a new infection.
- Improved accuracy: It helps improve the accuracy of TB testing by reducing the number of false-positive results.
- Detection of latent TB: It can help detect latent TB infection in individuals who have been previously infected but are not currently showing symptoms.
Note: The two-step TST is typically recommended for individuals who have never been previously tested for TB or have not had a TST in the past year.
