Induration of 10 mm or greater on a TB skin test is considered positive in which population?

• A. Recent immigrants, health care workers, homeless, IV drug users, and residents from high risk congregate settings
• B. Immunocompromised individuals
• C. Healthy adults without risk factors
• D. Children under 5 years old

Answer: (A)

TB skin test results are interpreted by how large the induration is, and the threshold for positivity depends on how likely a person is to have been infected or to progress to disease. In people with higher risk of TB exposure or progression—such as recent immigrants from areas with high TB prevalence, health care workers, people who are homeless, IV drug users, and residents of high-risk congregate settings—a 10 mm or greater induration is considered positive. This reflects their greater pretest probability of infection and the goal of identifying latent TB infection early to prevent active disease.

For context, other groups use different thresholds: immunocompromised individuals have a lower threshold (often 5 mm) for a positive result, and healthy adults with no risk factors typically require a larger induration (commonly 15 mm) to be considered positive. Children under 5 can also have a lower threshold in many guidelines, but the key idea remains that the test interpretation adjusts with risk to improve diagnostic accuracy.