The interpretation of a TB test (specifically the Mantoux tuberculin skin test) isn't defined by a single "normal range," but rather by the size of the induration (raised, hardened area) in millimeters, and this interpretation varies depending on individual risk factors. A negative TB test generally indicates no infection, while a positive test suggests TB infection.
Here's a breakdown of how induration size is interpreted:
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Negative Result: Generally, an induration of less than 5 mm is considered negative.
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Positive Result (Cutoff points and Risk Factors): The cutoff point for a positive test varies based on risk factors:
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≥ 5 mm: 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)
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≥ 10 mm: 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., prisons, nursing homes, homeless shelters, hospitals)
- Mycobacteriology laboratory personnel
- Persons with clinical conditions that place them at high risk (e.g., diabetes, prolonged corticosteroid therapy, leukemia, end-stage renal disease, chronic malabsorption syndromes, low body weight)
- Children younger than 4 years old
- Infants, children, and adolescents exposed to adults in high-risk categories
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≥ 15 mm: Considered positive in:
- Any person, including persons with no known risk factors for TB
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Important Considerations:
- This interpretation applies to the Mantoux tuberculin skin test (TST). Other TB tests, like Interferon-Gamma Release Assays (IGRAs) such as QuantiFERON-TB Gold or T-SPOT.TB, have different interpretation criteria (positive, negative, or indeterminate) determined by the laboratory performing the test.
- A positive TST or IGRA only indicates TB infection (latent or active), not necessarily active TB disease. Further evaluation (chest X-ray, sputum cultures) is needed to rule out active TB.
- False-positive TST results can occur in individuals who have received the BCG vaccine.
- False-negative TST results can occur in individuals who are immunosuppressed or recently infected.
Therefore, a simple "normal range" doesn't exist for the TB skin test. The interpretation depends heavily on the individual's risk factors and medical history, with a healthcare professional making the final determination.
