What is considered a positive result for PPD testing in individuals with no risk factors?

A positive result for PPD testing, or the Mantoux test, indicates exposure to the tuberculosis (TB) bacterium. The measurement of induration—measured in millimeters—at the injection site is crucial in determining the result.

For individuals with no risk factors for TB, a positive test is defined as an induration of 15 mm or greater. This threshold is based on the understanding that individuals who are healthy and do not have a known risk of TB exposure should show a more substantial skin reaction to indicate an infection.

The cut-off of 15 mm has been established by health authorities, including the Centers for Disease Control and Prevention (CDC), as the standard for screening individuals with no risk factors. These factors include being immunocompromised, having close contact with someone diagnosed with TB, or having a history of certain health conditions that increase the risk for developing TB. In the absence of such risk factors, lower thresholds of induration do not denote a significant reaction, thereby reinforcing that a measurement of 15 mm or more is required for a positive result in this specific population.