What is the minimum induration diameter in millimeters indicating a positive tuberculosis skin test in an HIV-positive patient?

• A. 0 millimeters
• B. 5 millimeters
• C. 10 millimeters
• D. 15 millimeters

Answer: (B)

In tuberculin skin testing, the size of the induration used to call a test positive depends on the person’s immune status. HIV infection weakens cell-mediated immunity, so the body's reaction to TB antigens can be smaller even when infection is present. Because of this, the cutoff is lowered to catch true infections in this high-risk group. A 5 mm or greater induration is considered a positive test in HIV-positive individuals, signaling the need for further evaluation for latent or active tuberculosis. Larger cutoffs are used in healthier or lower-risk populations, where a bigger reaction is required to indicate infection.