The two primary methods for tuberculosis (TB) screening are the Mantoux tuberculin skin test (TST) and the Interferon Gamma Release Assay (IGRA) blood test.
These tests help determine if a person has been infected with the bacteria that cause TB. It's important to remember that a positive test only indicates infection, not necessarily active TB disease. Further tests are required to confirm active disease.
1. Mantoux Tuberculin Skin Test (TST)
The Mantoux tuberculin skin test, often simply called the TB skin test, is administered by injecting a small amount of tuberculin (a purified protein derivative or PPD) under the skin of the forearm.
- How it works: The body's immune system, if previously exposed to TB bacteria, will react to the injected tuberculin, causing a localized swelling or induration.
- Reading the test: A trained healthcare professional must read the test 48-72 hours after administration. The size of the induration (raised area) is measured in millimeters. The interpretation of a positive result depends on individual risk factors, such as contact with TB patients, HIV status, and other medical conditions.
- Limitations: The TST can have false-positive results in individuals who have received the BCG (Bacille Calmette-Guérin) vaccine, which is used in many countries to prevent TB, or who have been infected with non-tuberculosis mycobacteria. It also requires a return visit for reading, which can be a barrier for some individuals.
2. Interferon Gamma Release Assay (IGRA)
Interferon Gamma Release Assays (IGRAs) are blood tests that measure how the immune system reacts to TB bacteria.
- How it works: A blood sample is taken and mixed with antigens (substances that can trigger an immune response) from TB bacteria. The test measures the amount of interferon-gamma (IFN-γ), a cytokine released by immune cells, in response to these antigens.
- Types of IGRAs: Common IGRAs include QuantiFERON-TB Gold In-Tube test (QFT-GIT) and T-SPOT.TB test.
- Advantages: IGRAs are not affected by prior BCG vaccination and generally require only one patient visit. They also may be preferred for individuals who are unlikely to return for TST reading.
- Limitations: IGRAs can be more expensive than TSTs and may have limitations in certain populations, such as young children.
Choosing the Right Test
The selection of the appropriate TB screening test depends on individual risk factors, medical history, and logistical considerations. Healthcare providers carefully assess these factors to determine the most suitable test for each patient. It's crucial to consult with a healthcare professional to determine which test is right for you.
