The interpretation of the TST is based on the size of induration (swelling) at the site of injection, measured in millimeters. The threshold for a positive test varies depending on the individual’s risk factors: - ≥5 mm: Considered positive in HIV-infected persons, recent contacts of TB cases, persons with fibrotic changes on chest radiograph consistent with prior TB, organ transplant recipients, and other immunosuppressed patients. - ≥10 mm: Considered positive in recent immigrants from high-prevalence countries, injection drug users, residents and employees of high-risk congregate settings, mycobacteriology laboratory personnel, persons with clinical conditions that place them at high risk, children under 4 years of age, or children and adolescents exposed to adults in high-risk categories. - ≥15 mm: Considered positive in any person, including those with no known risk factors for TB.
