An adolescent has a Mantoux skin test with 10 mm induration after 48 hours and no risk factors for TB. What should the clinician do next?

An adolescent has a Mantoux skin test with 10 mm induration after 48 hours and no risk factors for TB. What should the clinician do next?

• A. Ask about exposure to homeless persons.
• B. Order a chest radiograph to rule out active TB.
• C. Reassure the adolescent that this is a negative screen.
• D. Refer the adolescent to an infectious disease specialist.

Answer: (C)

Interpreting the Mantoux result hinges on both the size of the induration and the person’s risk factors for TB. For someone with no known TB risk factors, a positive threshold is 15 millimeters or more. A 10 millimeter induration, read at 48 hours, does not meet that cutoff, so it’s considered negative. In a low-risk adolescent like this, no further workup—such as a chest radiograph—or specialist referral is indicated, and reassure them accordingly. If risk factors or symptoms appeared later (for example, recent exposure to TB, immunosuppression, or immigration from a high-prevalence area), the interpretation would shift and might prompt additional evaluation. The key idea is that the cutoff for a positive test depends on risk factors, not a universal number.

Interpreting the Mantoux result hinges on both the size of the induration and the person’s risk factors for TB. For someone with no known TB risk factors, a positive threshold is 15 millimeters or more. A 10 millimeter induration, read at 48 hours, does not meet that cutoff, so it’s considered negative. In a low-risk adolescent like this, no further workup—such as a chest radiograph—or specialist referral is indicated, and reassure them accordingly. If risk factors or symptoms appeared later (for example, recent exposure to TB, immunosuppression, or immigration from a high-prevalence area), the interpretation would shift and might prompt additional evaluation. The key idea is that the cutoff for a positive test depends on risk factors, not a universal number.