In a tuberculin skin test, what finding is used to define a positive reaction?

• A. Induration size in millimeters
• B. Erythema size in millimeters
• C. Temperature change
• D. Color change of the skin

Answer: (A)

The essential idea is that a tuberculin skin test is interpreted by the size of the induration, a firm, raised swelling at the injection site. After injecting PPD intradermally, you read the result 48 to 72 hours later and define a positive reaction by measuring the induration in millimeters. Erythema (redness) can occur but does not define a positive result, and changes like temperature or color are not used to interpret the test. The exact induration threshold varies with risk factors, but the key concept is that positivity is based on the measurable induration, not the redness or other skin changes.