The diagnosis of TB skin infection, also known as latent tuberculosis infection, is primarily done through a Tuberculin Skin Test (TST), also called a Mantoux test. Here's how it's performed:
How a TB Skin Test Works
The Tuberculin Skin Test (TST) involves injecting a small amount of tuberculin, a substance extracted from the tuberculosis bacteria, just under the skin on the inside of the forearm. This process is critical for identifying an immune response to tuberculosis.
Step-by-Step Process:
- Injection: A health care worker injects a tiny amount of tuberculin into the skin.
- Waiting Period: The patient must wait for 48 to 72 hours. It's important not to cover the injection site during this time, as it can interfere with the test.
- Reading the Test: A healthcare professional will examine the injection site. They are looking for an induration (raised area) of the skin, not just redness.
- Interpretation:
- The size of the raised area, measured in millimeters, is crucial in determining whether the test is positive or negative. A positive test indicates a possible TB infection.
- A positive test usually requires further testing, like chest X-rays and sputum cultures, to confirm the presence of an active or latent infection.
Here is a table to summarize the key points:
| Step | Description |
|---|---|
| 1. Injection | A small amount of tuberculin is injected under the skin on the forearm. |
| 2. Waiting period | Wait 48 to 72 hours for the reaction to develop. |
| 3. Reading the test | Healthcare worker checks for a raised area (induration) at the injection site. |
| 4. Interpretation | The size of the raised area determines if the test is positive or negative. |
Important Considerations
- Redness Alone Isn't Enough: The presence of redness alone is not an indicator of a positive result; the skin must be raised (indurated).
- Follow-up Required: A positive skin test does not automatically mean someone has active TB; additional testing is required to determine that.
- False Negatives: The TB skin test might have false negatives. This could occur in people with a compromised immune system or those who have been recently infected. Therefore, results must be interpreted by a qualified healthcare provider.
By following these steps, a healthcare professional can accurately diagnose whether a person has been exposed to TB, although it may not determine whether they have active TB.
