A definitive "normal" Mantoux size doesn't exist universally; interpretation depends on individual risk factors. However, in people with no known risk factors for tuberculosis (TB), an induration of less than 15 mm is generally considered negative. This means it's unlikely they have a TB infection.
Factors Influencing Mantoux Test Interpretation:
The size threshold for a positive Mantoux test (indicating possible TB infection) varies depending on individual risk factors. Here's a breakdown:
- ≥ 5 mm: Considered positive in:
- HIV-infected persons
- Recent contacts of TB cases
- Persons with fibrotic changes on chest radiograph consistent with prior TB
- Patients with organ transplants
- Immunosuppressed patients (e.g., taking TNF-alpha antagonists or high-dose corticosteroids)
- ≥ 10 mm: Considered positive in:
- Recent immigrants (within the last 5 years) from high-prevalence countries
- Injection drug users
- Residents and employees of high-risk congregate settings (e.g., prisons, nursing homes, homeless shelters)
- Mycobacteriology lab personnel
- Persons with conditions that increase the risk of progressing to TB disease (e.g., diabetes, silicosis, chronic renal failure, certain malignancies)
- Children < 4 years old
- Infants, children, and adolescents exposed to adults in high-risk categories
- ≥ 15 mm: Considered positive in:
- Persons with no known risk factors for TB
Understanding Induration:
The Mantoux test involves injecting a small amount of tuberculin (purified protein derivative or PPD) under the skin. A healthcare professional must assess the reaction 48-72 hours later. The induration, a raised, hardened area, is measured in millimeters. Redness without induration is not considered positive.
Important Considerations:
- A positive Mantoux test doesn't necessarily mean someone has active TB disease. It indicates a TB infection, which could be latent (inactive) or active. Further testing, such as a chest X-ray and sputum cultures, is needed to determine if active TB disease is present.
- The Mantoux test is not perfect. False-positive and false-negative results can occur. Factors such as prior BCG vaccination, incorrect administration of the test, or weakened immune systems can affect the results.
- IGRA (Interferon-Gamma Release Assay) blood tests are another option for TB testing. They are not affected by prior BCG vaccination and might be preferred in certain populations.
Conclusion:
While an induration of less than 15mm is generally considered negative in low-risk individuals, the specific interpretation of a Mantoux test depends heavily on a person's individual risk factors and medical history. Consulting with a healthcare professional is essential for accurate interpretation and appropriate follow-up.
