From the Guidelines
The protocol for interferon-gamma release assays (IGRAs) for tuberculosis testing involves collecting a blood sample from the patient, which is then exposed to TB-specific antigens in specialized collection tubes, with the two main FDA-approved IGRAs being QuantiFERON-TB Gold Plus and T-SPOT.TB, as recommended by the CDC guidelines updated in 2010 1.
Key Steps in the Protocol
- For QuantiFERON testing, blood is collected in specific tubes containing TB antigens, incubated for 16-24 hours at 37°C, and then analyzed for interferon-gamma production using an ELISA method.
- For T-SPOT.TB, peripheral blood mononuclear cells are isolated, counted, and placed in wells with TB antigens, then incubated for 16-20 hours before measuring interferon-gamma producing cells.
Interpretation of Results
- Results are typically reported as positive (indicating TB infection), negative, or indeterminate, with interpretation criteria varying between the different FDA-approved IGRAs, such as QuantiFERON-TB Gold In-Tube test (QFT-GIT) and T-SPOT.TB, as outlined in the guidelines 1.
Clinical Considerations
- IGRAs are preferred over tuberculin skin tests for people who have received BCG vaccination since IGRAs are not affected by prior BCG vaccination.
- These tests detect TB infection but cannot distinguish between latent TB infection and active TB disease, so additional clinical evaluation including symptoms assessment, chest X-ray, and sputum testing may be needed for patients with positive IGRA results to determine if they have active disease requiring full treatment.
Important Considerations for Test Selection and Quality Control
- The choice of IGRA may depend on various factors, including the specific population being tested and the availability of resources, with the CDC guidelines providing recommendations for test selection and quality control 1.
- Quality control measures are essential to ensure the accuracy and reliability of IGRA results, including the use of standardized protocols and trained laboratory personnel.
From the Research
Interferon Test Protocol for TB
The protocol for interferon test for TB involves the use of interferon gamma release assays (IGRAs) to diagnose latent tuberculosis infection (LTBI) and active tuberculosis (TB) disease.
- The IGRAs are cell-mediated immune-based blood tests that measure the release of interferon-gamma in response to TB antigens 2.
- There are two main types of IGRAs: QuantiFERON-TB Gold In-Tube (QFT-G-IT) and T-SPOT.TB 3, 4.
- The tests involve collecting blood samples, incubating them with TB antigens, and measuring the release of interferon-gamma using an enzyme-linked immunosorbent assay (ELISA) or an enzyme-linked immunospot assay (ELISpot) 3, 4.
Interpretation of Results
The interpretation of IGRA results is based on the level of interferon-gamma released in response to TB antigens.
- A positive result indicates that the individual has been infected with TB and has mounted an immune response 2, 3, 4.
- A negative result indicates that the individual has not been infected with TB or has not mounted an immune response 2, 3, 4.
- The level of interferon-gamma released can also be used to estimate the risk of progression to active TB disease, with higher levels indicating a higher risk 5.
Sources of Variability
There are several sources of variability that can affect IGRA results, including:
- Preanalytical factors, such as blood collection and handling procedures 6.
- Analytical factors, such as the type of assay used and the laboratory procedures employed 6.
- Postanalytical factors, such as the interpretation of results and the reporting of findings 6.
- Manufacturing factors, such as the quality of the assay kits and reagents used 6.
- Immunological factors, such as the individual's immune status and the presence of other infections or diseases 6.