Diagnosis of skin TB primarily involves detecting the presence of Mycobacterium tuberculosis infection, often initiated through a Tuberculin Skin Test (TST). This test is a fundamental method used to determine if an individual has been exposed to the bacteria that cause tuberculosis, which can manifest in various forms, including skin TB.
Understanding the Tuberculin Skin Test (TST)
The Tuberculin Skin Test, also known as the Mantoux test, is a widely used diagnostic procedure for tuberculosis infection. It works by checking for an immune response in your body to tuberculin, a purified protein derivative from the TB bacterium. A positive reaction indicates that you have been infected with Mycobacterium tuberculosis, although it doesn't differentiate between latent TB infection and active TB disease.
How the TST is Performed
The procedure for a TST is straightforward and involves a few key steps:
- Step 1: Tuberculin Injection
A tiny amount of a substance called tuberculin is injected just below the skin on the inside of one forearm. This injection is very superficial, creating a small bump or wheal. - Step 2: Waiting Period
After the injection, you must wait for a specific period for your body to react. This waiting period is crucial, as the immune response takes time to develop. Within 48 to 72 hours, a health care worker will need to check your arm. - Step 3: Reading the Results
Upon returning to the healthcare facility, a trained professional will examine the injection site. They specifically look for any swelling at the injection site. The size of the raised skin (induration), not just redness, is meticulously measured in millimeters to determine whether the test is positive or negative.
TST Procedure Summary
For clarity, here's a summary of the Tuberculin Skin Test procedure:
| Aspect | Description |
|---|---|
| Substance Used | Tuberculin (a purified protein derivative) |
| Injection Site | Just below the skin on the inside of one forearm |
| Reading Time | Within 48 to 72 hours after injection |
| Result Basis | A health care worker measures the size of the raised, firm swelling (induration) at the injection site to determine a positive or negative result. Redness alone is not considered for interpretation. |
Importance of TST for Skin TB Detection
While a TST indicates the presence of TB infection rather than specifically pinpointing skin TB, a positive result is a critical diagnostic clue. It prompts further investigation to determine if the infection has progressed to active disease and its specific manifestation, such as cutaneous (skin) tuberculosis. Therefore, the TST serves as an important initial screening tool in the diagnostic pathway for skin TB, signaling the need for more targeted clinical and laboratory evaluations.
