In TB screening, an induration of 15 mm or greater is considered positive for individuals with no known risk factors.

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Interpreting the tuberculin skin test depends on how likely someone is to have TB infection to begin with. In people who have no known risk factors, you use a higher induration threshold to call the test positive. An induration of 15 millimeters or more is considered positive in this low-risk group because setting the cutoff higher helps keep false positives down when the pre-test probability of infection is low. Smaller indurations in these individuals are more likely to be false positives from non-specific skin reactions or prior vaccination, rather than true latent TB infection.

In contrast, for individuals with risk factors such as recent exposure, immunocompromise, or coming from areas with higher TB prevalence, a smaller induration is used to define positivity, because their pre-test probability of true infection is higher and missing a true infection has greater consequence. So, the 15-millimeter threshold applies specifically to those with no known risk factors, making it the best answer in this scenario.