Is Quantiferon Gold (Interferon Gamma Release Assay) the same as an Interferon Gamma Release Assay (IGRA)?

QuantiFERON-TB Gold is a Type of Interferon Gamma Release Assay (IGRA)

Yes, QuantiFERON-TB Gold is a specific brand of Interferon Gamma Release Assay (IGRA). QuantiFERON-TB Gold represents one type of IGRA that measures the release of interferon-gamma in blood samples when exposed to specific Mycobacterium tuberculosis antigens 1.

Understanding IGRAs and QuantiFERON-TB Gold

IGRAs are blood tests that detect the release of interferon-gamma (IFN-γ) from sensitized T cells when exposed to specific M. tuberculosis antigens. There are two main types of IGRAs:

1. QuantiFERON-TB Gold tests - These measure the amount of IFN-γ released using an enzyme-linked immunosorbent assay (ELISA) method 1, 2

   • Original QuantiFERON-TB test (QFT) - now discontinued
   • QuantiFERON-TB Gold (QFT-G)
   • QuantiFERON-TB Gold In-Tube (QFT-GIT)
   • QuantiFERON-TB Gold Plus (QFT-Plus) - the newest version 3

2. T-SPOT.TB - Uses the ELISPOT method to enumerate cells releasing IFN-γ 2, 4

How QuantiFERON-TB Gold Works

QuantiFERON-TB Gold tests detect the release of interferon-gamma in fresh heparinized whole blood from sensitized persons when incubated with specific M. tuberculosis antigens:

• Uses synthetic peptides representing M. tuberculosis-specific proteins such as early secretory antigenic target-6 (ESAT-6) and culture filtrate protein-10 (CFP-10) 1, 2
• These proteins are present in all M. tuberculosis but absent from BCG vaccine strains and most nontuberculous mycobacteria (except M. kansasii, M. szulgai, and M. marinum) 1
• The newer QFT-Plus has two TB antigen tubes: TB1 (stimulates CD4+ T cells) and TB2 (stimulates both CD4+ and CD8+ T cells) 3

Advantages of IGRAs Over Tuberculin Skin Tests (TST)

• Results available within 24 hours without requiring a return visit 1, 2
• Not subject to reader bias or interpretation errors 1, 2
• Higher specificity in BCG-vaccinated individuals 2
• Does not trigger an anamnestic response (boosting) 1
• More specific for M. tuberculosis than tests using purified protein derivative (PPD) 1

Limitations of IGRAs

• Requires proper blood collection and handling 2
• May have higher costs than TST (though potentially offset by reduced false positives) 2
• Cannot distinguish between latent TB infection and active TB disease 2
• Indeterminate results can occur, particularly in immunocompromised patients 2

Clinical Applications

The CDC recommends that IGRAs like QuantiFERON-TB Gold may be used in all circumstances in which the TST is currently used, including:

• Contact investigations
• Evaluation of recent immigrants
• Sequential-testing surveillance programs for infection control (e.g., healthcare workers) 1

IGRAs are particularly preferred for:

• BCG-vaccinated individuals
• Healthcare workers
• Children under 5 years of age
• Populations with low rates of return for TST reading 2

Evolution of QuantiFERON Tests

The QuantiFERON tests have evolved over time:

• Original QFT (approved 2001, now discontinued) - used PPD as antigen 1
• QFT-G (approved 2005) - uses ESAT-6 and CFP-10 antigens 1
• QFT-GIT - improved version with antigens in specialized tubes 2
• QFT-Plus - newest version with additional TB2 tube to capture CD8+ T cell responses 3

Comparative studies show good agreement between newer versions, with QFT-Plus potentially offering advantages for patients with active TB, recent TB, and specific immune conditions involving CD8+ T-cell responses 3.