>15 mm induration is positive for TB in individuals with no known risk. Which option best describes this criterion?

• A. HIV infection
• B. Healthcare workers
• C. No risk
• D. More than 15 mm

Answer: (D)

In interpreting the TB skin test, the cutoff for a positive result depends on how much risk a person has for TB infection. For individuals with no known risk factors, only larger reactions are considered positive to reduce false positives from prior vaccination or exposure to non-tuberculous mycobacteria. A positive result in this low-risk group is an induration of 15 mm or more. That’s why the option describing “More than 15 mm” best matches the criterion. In higher-risk groups, smaller indurations would be considered positive, but not for someone with no known risk.