A positive Mantoux test, also known as a tuberculin skin test (TST), is indicated by the size of the induration (a firm, raised area) at the injection site. The size that constitutes a positive result varies depending on individual risk factors. Generally, an induration of 15 mm or more is considered positive in individuals with no known risk factors for tuberculosis (TB). However, this threshold changes based on risk factors.
Different induration sizes are used as cutoffs for positivity based on risk factors:
-
≥ 5 mm is considered positive in:
- HIV-infected persons
- Recent contacts of a person with infectious TB disease
- Persons with fibrotic changes on chest radiograph consistent with prior TB
- Patients with organ transplants
- Persons who are immunosuppressed for other reasons (e.g., taking TNF-alpha antagonists)
-
≥ 10 mm is considered positive in:
- Recent immigrants (within 5 years) from high-prevalence countries
- Injection drug users
- Residents and employees of high-risk congregate settings (e.g., correctional facilities, nursing homes, homeless shelters, hospitals)
- Mycobacteriology laboratory personnel
- Persons with clinical conditions that place them at high risk (e.g., diabetes, silicosis, chronic renal failure, certain malignancies)
- Children < 4 years of age
- Infants, children, and adolescents exposed to adults in high-risk categories
-
≥ 15 mm is considered positive in:
- Persons with no known risk factors for TB.
- For employees who are otherwise at low risk for TB and who are tested as part of an infection control screening program at the start of employment, a reaction of ≥ 15 mm is considered positive.
Therefore, while a general rule of thumb is 15mm, it is critical to consider the individual's specific risk factors to correctly interpret the Mantoux test. Always consult with a healthcare professional for proper interpretation and follow-up.
