A positive Mantoux test, also known as a tuberculin skin test (TST), indicates potential exposure to tuberculosis (TB) bacteria. For employees at low risk for TB who are tested as part of a screening program at the start of employment, a reaction of ≥15 mm of induration is considered positive.
Understanding the Mantoux Test and Positive Results
The Mantoux test is a standard method for determining if a person is infected with Mycobacterium tuberculosis. Here's a breakdown of what a positive result means in the context of low-risk employees:
What is Induration?
Induration refers to a hard, raised area with clearly defined margins at the injection site. It's the key factor in interpreting the Mantoux test results. Redness alone is not considered a positive result.
Interpreting a Positive Result (≥15 mm)
For employees who are otherwise at low risk for TB and are tested at the start of employment:
- A positive Mantoux test result is defined as an induration of 15 mm or greater.
What Does a Positive Test Mean?
A positive Mantoux test suggests that the individual:
- May have been exposed to TB bacteria at some point.
- Could potentially have a latent TB infection (LTBI), where the bacteria are present in the body but are not causing active disease.
Why is the Threshold 15 mm for Low-Risk Individuals?
- The 15 mm threshold is used for individuals with a low likelihood of TB exposure to minimize false-positive results.
- In populations with a higher risk of TB exposure, smaller induration sizes (e.g., 5 mm or 10 mm) may be considered positive.
What Happens After a Positive Test?
A positive Mantoux test does not necessarily mean the person has active TB disease. Further evaluation is needed, which may include:
- Medical History and Physical Examination: To assess for symptoms of TB and risk factors.
- Chest X-ray: To look for signs of active TB disease in the lungs.
- Sputum Tests: If the chest X-ray is abnormal, samples of sputum (mucus coughed up from the lungs) may be tested for TB bacteria.
Mantoux Test Interpretation Table for Low-Risk Employees
| Induration Size | Interpretation |
|---|---|
| ≥15 mm | Positive (for low-risk employees tested at the start of employment) |
| Less than 15 mm | Negative |
