A 45-year-old man with recent close contact and tuberculosis exposure has a tuberculin skin test. Which induration measurement is the minimum induration for a positive test in this patient?

• A. 10 millimeters
• B. 5 millimeters
• C. 15 millimeters
• D. 2 millimeters

Answer: (B)

Interpreting a tuberculin skin test depends on the person’s risk for TB infection. In someone with recent close contact with an active TB case, the pre-test probability is high, so a smaller induration is considered positive. An induration of 5 millimeters or more is the threshold for positivity in this high-risk group, signaling possible latent TB infection that should be evaluated further. Larger cutoffs are used for lower-risk individuals: 10 millimeters for many moderate-risk groups, and 15 millimeters for those with no known risk factors. A 2-millimeter induration would almost never be considered positive.