Tuberculosis (TB) is a bacterial infection primarily targeting the lungs, caused by Mycobacterium tuberculosis. Screening for exposure uses the Tuberculin Skin Test (TST), also called the PPD test, which involves injecting a small amount of purified protein derivative under the skin. The standard two-step process is often required for individuals tested repeatedly, like healthcare workers, to establish an accurate immune response baseline. This procedure prevents misinterpretation of future screening results.
Understanding the 2-Step Test’s Purpose
The primary reason for the two-step TST is to account for the “boosting phenomenon.” In some individuals who were infected with TB a long time ago, the immune system’s memory response to the tuberculin solution can weaken over the years, leading to a false-negative result on the first test. The initial injection of the purified protein derivative acts as a stimulus, or “booster,” reawakening the immune system’s ability to react to the presence of the TB proteins.
If the first test is negative, but the second test shows a positive reaction, this indicates a boosted response from a past exposure. Without the second test, a positive result from a future one-step test could be incorrectly interpreted as a new infection. The two-step method identifies this boosted reaction immediately, establishing a reliable negative or positive baseline. This procedure is performed only once to determine the person’s baseline status.
The Required Timing Between Steps
The timing between the two TST steps is important to accurately capture the boosting phenomenon and avoid misdiagnosis. The second test must be administered within a specific window following the reading of the first negative test. This window is generally 7 to 21 days, or one to three weeks, after the first PPD was placed. Some guidelines allow the second test to be placed up to four weeks after the first.
Placing the second injection in less than seven days may not allow enough time for the immune system to mount a boosted response, potentially resulting in a false-negative test. Waiting too long, such as more than 12 months, necessitates restarting the entire two-step process. Adherence to the one-to-three-week period ensures the most accurate interpretation of the individual’s status.
Reading the Results and Follow-Up
Both the first and second TST injections must be read by a trained healthcare worker within 48 to 72 hours after placement. The reading involves assessing the injection site for induration, which is a raised, firm area of swelling, not just redness or bruising. The size of this induration, measured in millimeters, is the determining factor for a positive result.
The threshold for a positive reading depends on the individual’s risk factors, typically defined as five or ten millimeters of induration. If the second test is positive, the individual has a latent TB infection (LTBI) and requires clinical follow-up. This usually involves a chest X-ray and medical evaluation to determine if the infection has progressed to active TB disease. If the chest X-ray shows no signs of active disease, the person may be referred for LTBI treatment to prevent future progression.