What is classified as a positive PPD test in individuals not in high prevalence groups?

A positive PPD (purified protein derivative) test, which is used to screen for tuberculosis (TB) infection, is categorized based on the size of the induration, measured in millimeters, at the site of the injection. For individuals who are not in high-risk groups, a threshold of 15 mm is considered positive. This cutoff reflects the expectation that those without significant risk factors for TB exposure or disease are unlikely to have a significant response to the test below this measurement.

In low-risk populations, the immune response to TB is less likely to be heightened, and thus a larger induration is necessary to indicate a positive test. The other thresholds, such as 5 mm and 10 mm, are typically used for populations with higher risk factors such as recent contacts of known TB cases, immunocompromised individuals, or those with certain clinical conditions that predispose them to more severe TB disease.