What is a TB Skin Test?
A TB skin test, also known as a Mantoux tuberculin skin test or PPD (purified protein derivative) test, is a simple procedure used to screen for Mycobacterium tuberculosis, the bacteria that causes tuberculosis (TB). It checks if you've been exposed to the TB bacteria, not if you currently have active TB disease.
The test involves injecting a small amount of TB protein (antigen) just under the skin of your forearm. Your immune system reacts to this protein if you've been exposed to TB bacteria in the past. This reaction is then checked 48-72 hours later by a healthcare professional who measures the size of any swelling or induration (hardening) at the injection site. A larger area of induration usually indicates a positive result, though interpretation depends on factors like individual risk and exposure history. A positive result doesn't necessarily mean you have active TB, but further testing is needed to determine if you have latent TB infection (LTBI) or active TB disease.
Types of TB Tests
There are two main types of tests for TB infection:
- TB Skin Test: This is the test described above. The Mantoux method is the most common technique for administering the skin test.
- TB Blood Test: An alternative method that detects antibodies against TB bacteria in your blood.
Two-Step TB Skin Test
A two-step TB skin test is sometimes used to reduce the chances of misinterpreting a boosted reaction from an old TB infection as a recent infection. This involves two separate skin tests administered several weeks apart.
Interpreting Results
The interpretation of a TB skin test requires professional medical judgment and considers several factors including:
- Size of induration: The size of the hardened area at the injection site.
- Risk factors: Exposure to individuals with TB, weakened immune system, etc.
- Medical history: Previous TB infections or vaccinations.
A healthcare professional will determine the next steps based on the test results and the individual's risk profile. A positive result will usually lead to further tests such as chest X-rays and sputum tests to confirm whether active TB disease is present.
