A Mantoux test yields an induration of 10 mm. What is the most accurate interpretation?

A Mantoux test yields an induration of 10 mm. What is the most accurate interpretation?

• A. Active disease is present.
• B. She has been exposed to M. tuberculosis or has been vaccinated with bacille Calmette-Guérin.
• C. The disease is not active, but preventive treatment should be initiated.
• D. The reaction is questionable and should be repeated.

Answer: (D)

The key idea is that interpreting a Mantoux reaction depends on risk context, not just a fixed number. An induration of 10 mm is not automatically diagnostic of latent TB in someone without clear risk factors. It can reflect past BCG vaccination, a non-specific reaction, or borderline infection. Because this result is ambiguous, the best next step is to repeat the test or pursue additional evaluation (such as an interferon-gamma release assay and a chest X-ray) to distinguish latent infection from no infection or active disease. Only with a clearer positive result and no signs of active disease would preventive treatment be initiated. If there are strong risk factors (recent contact, immunosuppression, etc.), a 10 mm induration might be considered positive, which would change management. This is why the reaction is described as questionable and warrants re-testing.

The key idea is that interpreting a Mantoux reaction depends on risk context, not just a fixed number. An induration of 10 mm is not automatically diagnostic of latent TB in someone without clear risk factors. It can reflect past BCG vaccination, a non-specific reaction, or borderline infection. Because this result is ambiguous, the best next step is to repeat the test or pursue additional evaluation (such as an interferon-gamma release assay and a chest X-ray) to distinguish latent infection from no infection or active disease. Only with a clearer positive result and no signs of active disease would preventive treatment be initiated. If there are strong risk factors (recent contact, immunosuppression, etc.), a 10 mm induration might be considered positive, which would change management. This is why the reaction is described as questionable and warrants re-testing.