Tuberculosis (TB) is caused by the bacterium Mycobacterium tuberculosis, primarily affecting the lungs. Screening tests determine if a person has been exposed and developed a latent infection, meaning the bacteria are present but not causing active symptoms. The standard method is the Tuberculin Skin Test (TST), also known as the Mantoux or PPD test, which involves injecting purified protein derivative (PPD) under the skin. The two-step TST is a variation designed to establish an accurate baseline status for individuals who require routine testing.
Identifying the Booster Phenomenon
The two-step testing protocol exists to circumvent the “booster phenomenon.” An individual infected with TB years ago, or who previously received the Bacille Calmette-Guérin (BCG) vaccine, may have a diminished immune response to a single TST. This waning immunity can cause the initial test to yield a false-negative result, despite previous exposure.
The first PPD injection acts as an immune stimulus, reminding the immune system of the previous exposure. This stimulation increases the body’s ability to react to the tuberculin protein. A second test administered afterward results in a “boosted” positive reaction, accurately reflecting the old infection status.
Failing to perform the two-step test on someone with a remote infection history can lead to misinterpretation during future screenings. If a single TST later causes a positive result due to the booster effect, it might be incorrectly interpreted as a new TB infection or “conversion.” The two-step method is used for individuals requiring baseline testing, such as new employees in healthcare or correctional facilities.
The Step-by-Step Testing Procedure
The two-step TST requires multiple visits to a healthcare provider. The process begins with the first TST, where a small amount of PPD solution is injected into the top layer of skin, usually on the inner forearm. This injection aims to create a pale elevation of the skin, known as a wheal.
The individual must return to the clinic 48 to 72 hours after the first injection for the reading. A trained provider measures the induration—the firm, raised swelling at the site, not the redness—and records the measurement in millimeters.
If the first TST result is positive, the process concludes, and the individual is considered infected. If the first test is negative, a mandatory waiting period follows before the second test is administered. This period is generally between one and four weeks after the first injection, allowing the booster phenomenon to occur.
During the second visit, the second TST is administered on the opposite forearm or a different site. This injection uses the same PPD solution and technique as the first. The individual returns 48 to 72 hours later to have the second site read, completing the four-visit process to establish the baseline status.
What Your Test Results Mean
The interpretation of the two-step TST results determines the individual’s baseline status regarding TB infection. If both the first and second TST injections yield a negative result, the individual is considered uninfected with Mycobacterium tuberculosis. This negative baseline is documented for use in future routine screenings.
If the first TST is positive, the two-step process concludes immediately, and the person is classified as having a TB infection. This result confirms the presence of the bacteria in a latent or active state, as the immune system reacted strongly to the tuberculin.
A positive result from the second TST, following a negative first test, is considered a “boosted” reaction. This is interpreted as an old TB infection, not a new one. Any positive result requires follow-up medical evaluation, typically including a symptom review and a chest X-ray. This determines if the infection is latent or active, and may lead to treatment to prevent active disease.