In low-risk individuals, what induration size on a Mantoux test indicates a positive result?

In low-risk individuals, what induration size on a Mantoux test indicates a positive result?

• A. 5 mm or more
• B. 10 mm or more
• C. 15 mm or more
• D. 20 mm or more

Answer: (C)

In interpreting a Mantoux tuberculin skin test, the key idea is that the induration size is read against the person’s risk category. The test measures delayed-type hypersensitivity to purified protein derivative, and you read the size of the induration in millimeters 48 to 72 hours after administration. For individuals with no known risk factors, a larger induration is needed to indicate true infection and to avoid false positives from prior BCG vaccination or exposure to non-tuberculous mycobacteria. Therefore, an induration of 15 mm or more is considered positive in low-risk people. Smaller indurations are less specific in this group and could reflect non-TB factors, which is why the threshold is higher here. In higher-risk groups, smaller thresholds are used (5 mm for the highest risk, 10 mm for moderate risk), and a 20 mm threshold is not the standard cut-off for this population.

In interpreting a Mantoux tuberculin skin test, the key idea is that the induration size is read against the person’s risk category. The test measures delayed-type hypersensitivity to purified protein derivative, and you read the size of the induration in millimeters 48 to 72 hours after administration. For individuals with no known risk factors, a larger induration is needed to indicate true infection and to avoid false positives from prior BCG vaccination or exposure to non-tuberculous mycobacteria. Therefore, an induration of 15 mm or more is considered positive in low-risk people. Smaller indurations are less specific in this group and could reflect non-TB factors, which is why the threshold is higher here. In higher-risk groups, smaller thresholds are used (5 mm for the highest risk, 10 mm for moderate risk), and a 20 mm threshold is not the standard cut-off for this population.