When is a 5-mm or greater induration on PPD testing considered positive?

A 5-mm or greater induration on tuberculin skin testing (PPD testing) is considered positive primarily in individuals who are immunocompromised, including those who are HIV-infected. This threshold is established because these individuals have a significantly higher risk of progression to active tuberculosis (TB) following infection and therefore require more sensitive criteria for a positive test result.

In the context of immunocompromised populations, a smaller amount of induration indicates a potential infection as their immune response may not be as robust as that in healthy individuals. This is critical in managing public health, as it helps identify cases of latent TB infection that could otherwise lead to more severe disease.

The determination of positivity based on different criteria takes into account the patient's overall risk profile and previous exposure to TB. For example, different cutoffs are used for high-risk groups, recent contacts of TB cases, and individuals with certain underlying health conditions, ensuring that those most at risk receive the appropriate evaluation and management.