The term “TB stimulated gamma interferon” refers to the principle behind modern blood tests for tuberculosis (TB). These tests, called Interferon-Gamma Release Assays (IGRAs), detect an immune response to the TB bacterium. They work by measuring the amount of gamma interferon, a substance released by immune cells, to determine if a person has been exposed to the infection.
The Immune System’s Response to Tuberculosis
When the bacterium Mycobacterium tuberculosis enters the body, it triggers a defense from the immune system. A type of white blood cell called a T-cell is central to this response. Upon encountering the bacteria, these T-cells become sensitized, meaning they are primed to recognize and react to the invader if they encounter it again.
Once sensitized, these T-cells orchestrate the body’s defense by releasing signaling proteins called cytokines. Gamma interferon is a cytokine that plays a primary role in the fight against TB. It acts as a messenger, activating other immune cells, such as macrophages, which are responsible for engulfing and destroying pathogens.
The release of gamma interferon is a specific and measurable indicator that a person’s T-cells have been exposed to M. tuberculosis. This biological process allows for the detection of a TB infection, even when it is not causing active symptoms. The memory of this encounter is stored within the T-cells.
How the Gamma Interferon Blood Test Works
The Interferon-Gamma Release Assay (IGRA) is a laboratory test that begins with a blood draw. This blood sample contains the patient’s immune cells, including the T-cells that are the focus of the test.
In the laboratory, the blood is mixed with specific antigens. These antigens are proteins, such as ESAT-6 and CFP-10, unique to the Mycobacterium tuberculosis bacterium and absent from the BCG vaccine strain. Control tubes are used to ensure the test is valid and the immune cells are functioning correctly.
If the patient’s T-cells have been sensitized to TB, they will recognize these antigens and release gamma interferon. After an incubation period of 16 to 24 hours, a machine measures the concentration of gamma interferon. A significant amount released in response to the TB antigens indicates a positive reaction.
Understanding Test Results
Test results are reported as positive, negative, or indeterminate. A positive result indicates the person’s immune system has been sensitized to the TB bacteria. However, the test cannot distinguish between a latent TB infection, where bacteria are dormant, and active TB disease, where bacteria are multiplying.
A negative result suggests that a TB infection is unlikely. An indeterminate result is inconclusive and can happen for several reasons, such as a problem with the blood sample or if the patient has a weakened immune system. This is because the T-cells may not react appropriately in the control tubes, making the results uninterpretable.
Further investigation is required to understand the context of the results. For instance, a person with a compromised immune system might have an indeterminate result because their T-cells cannot produce enough gamma interferon. The test’s outcome is one piece of the diagnostic puzzle.
Next Steps After a Positive Result
After a positive result, the next step is to determine if the person has a latent TB infection or active TB disease. This distinction dictates the course of treatment and any necessary public health measures.
The follow-up process includes a medical evaluation of symptoms and a physical examination. A chest X-ray is used to check for lung abnormalities characteristic of active TB. If the X-ray shows signs of disease, a sputum sample may be analyzed to confirm the presence of M. tuberculosis bacteria.
If the chest X-ray is normal and the person has no symptoms, the diagnosis is latent TB infection. Treatment involves a preventative course of antibiotics to eliminate the dormant bacteria and prevent them from becoming active in the future.
Comparison with the TB Skin Test
The gamma interferon blood test (IGRA) and the tuberculin skin test (TST) are the two primary methods for detecting a TB immune response. The IGRA requires only one visit for a blood draw, whereas the TST requires a second visit 48 to 72 hours later to have the skin reaction read.
A primary advantage of the IGRA is that its results are not affected by a prior Bacille Calmette-Guérin (BCG) vaccination. The BCG vaccine can cause a false-positive reaction to the TST, so the IGRA is more accurate for people who have been vaccinated.
The TST, however, has been in use for much longer and is often less expensive to administer. The choice between the tests depends on factors like a person’s vaccination history, age, and health status, as well as available healthcare resources.