QuantiFERON-TB Gold Test Overview
Yes, QuantiFERON-TB Gold (QFT-G) is an FDA-approved blood test that detects Mycobacterium tuberculosis infection by measuring interferon-gamma release in response to specific TB antigens. 1
What the Test Is
QuantiFERON-TB Gold is an enzyme-linked immunosorbent assay (ELISA) that detects interferon-gamma (IFN-γ) released from sensitized white blood cells when whole blood is incubated with synthetic peptides representing two M. tuberculosis proteins:
The test received final FDA approval on May 2,2005, as an aid for diagnosing both latent tuberculosis infection (LTBI) and active tuberculosis disease. 1
Key Technical Characteristics
The test uses specific antigens that provide greater specificity than traditional tuberculin skin testing (TST):
- ESAT-6 and CFP-10 are secreted by all M. tuberculosis and pathogenic M. bovis strains 1
- These proteins are absent from all BCG vaccine strains 1
- They are absent from most nontuberculous mycobacteria except M. kansasii, M. szulgai, and M. marinum 1
The test requires fresh heparinized whole blood and provides results within 24 hours without requiring a second patient visit. 1
Clinical Applications
According to CDC guidelines, QFT-G may be used in all circumstances where TST is currently used:
- Contact investigations 1
- Evaluation of recent immigrants 1
- Sequential-testing surveillance programs for infection control (e.g., healthcare workers) 1
Advantages Over Tuberculin Skin Test
The test offers several practical and clinical advantages:
- Not affected by prior BCG vaccination 2, 3
- No cross-reactivity with most nontuberculous mycobacteria 4, 3
- Requires only one patient visit (no return needed for reading) 2, 4
- No reader bias or placement errors inherent to skin testing 2
- Results available within 8-30 hours 2
Test Performance
The sensitivity of QFT-G is statistically similar to TST for detecting infection in persons with untreated culture-confirmed tuberculosis. 1
In active TB diagnosis, QFT-G demonstrated 86% sensitivity in patients with valid results, significantly higher than TST's 62% sensitivity. 5
Important Limitations
The CDC recommends against using QFT-G to monitor treatment response or confirm cure. 6
Indeterminate results can occur and are associated with:
- Lower lymphocyte counts (threshold <1.15 × 10⁹ cells) 7
- Lower CD4+ T cell counts (threshold <467.7-478.5 × 10⁶ cells) 7
- Detection of substantial IFN-γ in the nil control sample 1
Test Versions
QuantiFERON-TB Gold replaced the original QuantiFERON-TB test (QFT), which is no longer commercially available. 1
A newer version, QuantiFERON-TB Gold Plus (QFT-Plus), has since been developed with an additional TB antigen 2 (TB2) tube to induce CD8+ T cell responses, and may detect more LTBI cases in patients receiving long-term immunosuppressant therapy. 7
Dual Testing Considerations
Guidelines suggest considering dual testing with both IGRA and TST for patients at highest risk of progression to active TB, where a positive result from either test is considered positive. 2
This approach increases sensitivity at the cost of reduced specificity and may be particularly useful in severely immunocompromised patients. 2