What is an Interferon‑Gamma Release Assay (IGRA) test?

What is an IGRA Test?

An IGRA (Interferon-Gamma Release Assay) is a blood test that measures the immune system's response to Mycobacterium tuberculosis by detecting interferon-gamma released from T cells when exposed to specific TB antigens, used to diagnose latent TB infection and aid in diagnosing active TB disease. 1

How IGRAs Work

IGRAs are in vitro, T cell-based assays that measure interferon gamma (IFN-γ) release by sensitized T cells in response to highly specific Mtb antigens. 1 The test works by:

• Drawing whole blood and exposing it to synthetic peptides representing specific M. tuberculosis proteins, particularly early secretory antigenic target-6 (ESAT-6) and culture filtrate protein 10 (CFP-10) 1
• These proteins are present in all M. tuberculosis strains but are absent from BCG vaccine strains and most nontuberculous mycobacteria, making the test more specific than the tuberculin skin test (TST) 1, 2
• The test measures the amount of interferon-gamma produced by T cells that have been previously sensitized to TB antigens in vivo 3

FDA-Approved IGRA Tests

Two main IGRAs are currently FDA-approved in the United States 1:

QuantiFERON-TB Gold In-Tube (QFT-GIT)

• Blood (1 mL) is collected into three special tubes containing lyophilized antigens and controls 1
• One tube contains peptides representing ESAT-6, CFP-10, and TB7.7 1
• After 16-24 hours of incubation at 37°C, plasma is tested using ELISA to measure IFN-γ concentration 1
• Requires no additional sample handling due to in-tube methodology 1

T-SPOT.TB Test

• Uses peripheral blood mononuclear cells (PBMCs) incubated with ESAT-6 and CFP-10 peptides 1
• Uses an enzyme-linked immunospot assay (ELISpot) to detect the number of cells secreting IFN-γ (represented as spots) 1

Key Advantages Over TST

IGRAs offer superior specificity compared to the tuberculin skin test, particularly in BCG-vaccinated populations, because the test antigens are absent from BCG vaccine strains. 1, 4, 2 Additional advantages include:

• Only one patient visit required (no return visit for reading) 2, 5
• Not affected by prior BCG vaccination, reducing false-positive results 4, 2, 5
• No reader bias or errors in placement and interpretation 4
• More specific than TST due to use of M. tuberculosis-specific antigens 2, 5

Important Limitations and Caveats

IGRAs require viable white blood cells in fresh blood specimens, limiting their use to facilities with appropriate laboratory capabilities. 1 Critical limitations include:

• Cannot distinguish between latent TB infection and active TB disease 5, 6
• Sensitivity is reduced in immunocompromised patients, including those with HIV infection or severe TB disease 5, 6
• False-positive results can occur with M. kansasii, M. szulgai, and M. marinum infections due to shared antigens 1, 7
• Neither more sensitive than TST for detecting latent infection 5, 6
• Sensitivity and specificity vary across different clinical contexts and populations 2

Clinical Applications

The CDC recommends that IGRAs may be used in all situations where TST is recommended, including contact investigations, evaluation of recent immigrants, and serial-testing surveillance programs. 1, 2 Specific scenarios where IGRAs are preferred include:

• BCG-vaccinated populations where higher specificity is needed 4, 2
• One-time screening situations (immigrants, pre-biologic therapy) where return visits are problematic 4
• Combined use with TST in immunocompromised patients to increase sensitivity 2

However, the CDC recommends PPD (TST) for serial testing programs like healthcare worker surveillance because it can demonstrate clear conversion from negative to positive, indicating new infection. 4

Common Pitfalls to Avoid

• Do not use IGRAs as the sole diagnostic test for active TB disease—they lack sufficient sensitivity and specificity to exclude or confirm active TB 6
• Remember that positive IGRA results in patients with M. kansasii, M. marinum, or M. szulgai may represent cross-reactivity rather than true TB infection 1, 7
• Do not assume IGRAs are more sensitive than TST—both have similar sensitivity limitations, particularly in immunocompromised patients 5, 6
• Ensure fresh blood specimens reach the laboratory promptly, as viable white blood cells are required for accurate results 1