What indicates a positive Mantoux test in low-risk individuals?

What indicates a positive Mantoux test in low-risk individuals?

• A. Induration greater than or equal to 15 mm.
• B. Induration greater than or equal to 5 mm.
• C. Induration greater than or equal to 10 mm.
• D. Induration greater than or equal to 20 mm.

Answer: (A)

In the Mantoux test, positivity in someone with no known risk factors is defined by an induration of 15 mm or more. The induration—the firm, raised area formed by a delayed-type hypersensitivity reaction to the injected tuberculin—is measured 48–72 hours after administration; erythema alone isn’t used to interpret the result. This higher threshold helps prevent false positives from factors like prior BCG vaccination or exposure to non-tuberculous mycobacteria. A positive result indicates TB infection at some point, not necessarily active disease, so it warrants further evaluation (clinical assessment, chest imaging, and consideration of latent TB treatment if appropriate). Lower thresholds (such as 5–10 mm) are used for people at higher risk, which is why those cutoffs aren’t applicable to low-risk individuals.

In the Mantoux test, positivity in someone with no known risk factors is defined by an induration of 15 mm or more. The induration—the firm, raised area formed by a delayed-type hypersensitivity reaction to the injected tuberculin—is measured 48–72 hours after administration; erythema alone isn’t used to interpret the result.

This higher threshold helps prevent false positives from factors like prior BCG vaccination or exposure to non-tuberculous mycobacteria. A positive result indicates TB infection at some point, not necessarily active disease, so it warrants further evaluation (clinical assessment, chest imaging, and consideration of latent TB treatment if appropriate). Lower thresholds (such as 5–10 mm) are used for people at higher risk, which is why those cutoffs aren’t applicable to low-risk individuals.