Tuberculosis (TB) is a disease caused by the bacterium Mycobacterium tuberculosis that primarily affects the lungs. Screening for TB infection involves the Tuberculin Skin Test (TST), also known as the Mantoux test, which uses Purified Protein Derivative (PPD) to detect a person’s immune response to TB proteins. The standard TST is a single injection indicating exposure to the bacteria. The two-step TST is a modification designed to improve the accuracy of initial, or baseline, testing for specific groups.
The Purpose of the Two-Step Method
The two-step TST accounts for a biological phenomenon known as “boosting” or the “recall effect.” Individuals previously infected with TB or who received the Bacille Calmette-Guérin (BCG) vaccine may have a diminished immune response, causing a falsely negative result on the first TST. The PPD injection in the first test can stimulate the immune system’s memory of the old infection. The two-step method intentionally elicits this boosted reaction immediately to establish a true baseline reading. This prevents future routine screening tests from being misinterpreted as a new infection, avoiding unnecessary follow-up.
Step-by-Step Procedure
The procedure begins with the first TST, injecting PPD intradermally, usually on the inner forearm. A healthcare professional assesses the site 48 to 72 hours later, measuring the induration—the palpable, hardened area. Only the diameter of this firm swelling is recorded in millimeters; redness is not measured. If the first test is positive, the procedure stops, and the patient is classified as infected. If the first TST is negative, the second TST is administered between one and four weeks later (7 to 21 days is common), typically in the opposite forearm, and read 48 to 72 hours after placement.
Interpreting the Results
The two-step TST establishes a definitive baseline status regarding prior TB exposure. A positive reading on the first TST (often 10 millimeters or greater induration) immediately classifies the individual as having a latent TB infection, and the second test is unnecessary. If the first test is negative but the second test is positive, this is considered a boosted reaction, confirming a latent infection from the distant past. Any positive result requires follow-up evaluation, typically including a chest x-ray and physician assessment, to rule out active TB disease. Once a person has a documented positive two-step result, they should not receive future TSTs, as they are already known to be infected.
Who Needs the Two-Step Test
The two-step TST is recommended for initial (baseline) screening of adults who will be retested periodically. These populations are at a higher risk of exposure and are often subject to serial testing, such as annual screening. Establishing a reliable baseline from the outset ensures that any subsequent single TST conducted years later can be accurately interpreted as a new TB conversion rather than a delayed response.
Populations Requiring Baseline Screening
The two-step TST is typically required for new employees in:
- Healthcare settings
- Nursing homes
- Correctional facilities
- Long-term care facilities