What size induration from a mantoux test requires further diagnostic testing for TB in a person with no known risks?

The Mantoux test, also known as the tuberculin skin test, evaluates for exposure to the tuberculosis (TB) bacteria by measuring the induration that forms at the test site. In interpreting the size of the induration, different thresholds are used based on a person's risk factors for TB exposure and infection.

For individuals with no known risk factors, an induration of greater than 15 mm is generally considered significant. This threshold indicates that the person's immune system has shown a reaction strong enough to suggest a possible TB infection. Further diagnostic testing, such as a chest X-ray or a follow-up sputum test, would be warranted to confirm or rule out active TB disease.

In contrast, the lower induration sizes—greater than 5 mm or greater than 10 mm—are more relevant for persons with specific risk factors, such as HIV infection, recent close contact with an individual with TB, or other immunocompromising conditions. These populations may have a lower threshold for diagnostic testing. Therefore, in a person with no known risk factors, the correct size of induration that necessitates further evaluation for TB is actually greater than 15 mm.