What does the "booster effect" refer to in PPD skin testing?

The term "booster effect" in the context of PPD skin testing refers specifically to a delayed immune response that occurs when an individual who has previously been exposed to tuberculosis (TB) is re-tested with the PPD. In many cases, especially in those who have been previously sensitized (like individuals who had latent TB or were vaccinated with BCG), the first test may yield a weak or indeterminate reaction due to the fact that the immune system needs time to react.

When a person with prior TB exposure or sensitization undergoes a second PPD test, they might exhibit a significantly more robust immune response, resulting in a larger induration than what was observed during the initial test. This scenario demonstrates the "booster effect," which serves as a reminder that a single negative PPD test does not conclusively rule out TB infection in someone who may have had previous exposure.

Understanding this effect is crucial for healthcare providers when interpreting PPD skin test results, especially in populations at risk or those who have been previously vaccinated or exposed to TB, as it helps in making accurate assessments of TB infection status.