The Mantoux test, also known as a Pirquet test or tuberculin sensitivity test (TST), does not definitively confirm TB (tuberculosis); it indicates exposure to the tuberculosis bacteria.
Understanding the Mantoux Test
The Mantoux test is a diagnostic skin test that assesses the body's immunological response to tuberculosis. It involves injecting a small amount of tuberculin under the skin of the forearm.
Here's how it works:
- Injection: A purified protein derivative (PPD) of tuberculin is injected.
- Reaction: The injection site is checked after 48-72 hours for induration (a raised, hardened area).
- Interpretation:
- A positive reaction (specific size of induration) suggests the individual has been exposed to TB bacteria, but not necessarily that they have active TB.
- A negative reaction typically suggests the individual has not been exposed, though some individuals with TB may not show a reaction.
What a Positive Mantoux Result Means
A positive Mantoux test indicates that:
- The person has been infected with TB bacteria at some point in their life.
- Their immune system has mounted a response to the TB bacteria.
However, a positive test does not mean:
- The person has active TB disease.
- The person is contagious.
Further tests, like chest X-rays and sputum cultures, are required to confirm active TB.
What a Negative Mantoux Result Means
A negative Mantoux test:
- Typically suggests that the individual has not been exposed to the bacteria.
- Does not rule out TB. Certain circumstances may result in false-negative test results, such as:
- Being immunocompromised.
- Recent TB infection.
- Technical issues with the test administration or interpretation.
Mantoux Test Limitations
The Mantoux test has limitations:
- False positives: May occur due to previous TB vaccination or exposure to related bacteria.
- False negatives: As mentioned, certain factors can suppress the immune response, resulting in a false negative.
- Not diagnostic for active TB: Requires additional testing to confirm.
Summary
| Test Name | Purpose | Indicates Exposure? | Confirms Active TB? |
|---|---|---|---|
| Mantoux (TST) | Assess response to TB bacteria | Yes | No |
| Chest X-ray | Examine lung tissue | Maybe | Maybe |
| Sputum Culture | Identify TB bacteria in sputum | Maybe | Yes |
Conclusion
The Mantoux test is an important tool in identifying individuals who have been exposed to TB bacteria, but it is not sufficient on its own to diagnose active TB. Further tests are required.
