What is QuantiFERON-TB Gold?
QuantiFERON-TB Gold (QFT-G) is an FDA-approved blood test that detects Mycobacterium tuberculosis infection by measuring interferon-gamma release from white blood cells when exposed to TB-specific antigens (ESAT-6 and CFP-10). 1
Test Mechanism
The test works by incubating fresh whole blood with synthetic peptides representing two M. tuberculosis proteins:
- Early secretory antigenic target-6 (ESAT-6)
- Culture filtrate protein-10 (CFP-10)
These proteins are present in all M. tuberculosis strains but absent from BCG vaccine strains and most nontuberculous mycobacteria (except M. kansasii, M. szulgai, and M. marinum), making the test more specific than traditional tuberculin skin testing (TST) 1.
Key Advantages Over TST
Greater specificity: Does not cross-react with BCG vaccination or most environmental mycobacteria 1, 2
Single visit required: Unlike TST which requires return in 48-72 hours for reading, QFT-G results are available from one blood draw 1
No boosting effect: Serial testing does not cause boosting of subsequent results, eliminating interpretation uncertainty in surveillance programs 1
Objective results: Eliminates reader variability and subjectivity inherent in TST measurement 3
Clinical Applications
According to CDC guidelines, QFT-G may be used in all circumstances where TST is currently used 1:
- Contact investigations for infectious TB cases
- Evaluation of recent immigrants
- Serial surveillance programs (healthcare workers, correctional facilities)
- Screening before anti-TNF therapy or immunosuppression 4
Test Interpretation
Positive result: Indicates M. tuberculosis infection (either latent or active disease) - requires chest X-ray and clinical evaluation to exclude active TB before treating latent infection 1
Negative result: In healthy adults, unlikely to have TB infection. However, for recent TB contacts, repeat testing at 8-10 weeks post-exposure is recommended, similar to TST protocols 1
Indeterminate result: Provides no useful information about infection status. Options include repeating QFT-G with new specimen or administering TST. High-risk patients warrant repeat testing 1
Important Caveats
Sensitivity comparable to TST: In culture-confirmed TB patients, QFT-G sensitivity (65-87%) is statistically similar to TST 1, 5
Cannot distinguish latent from active disease: Like TST, QFT-G cannot differentiate between latent TB infection and active TB disease - clinical and radiographic evaluation remains essential 1
Predictive value unknown: The test's ability to predict progression to active TB disease has not been determined and would require years of prospective study 1
Indeterminate rates: Occur in 0-33% of cases, particularly in immunocompromised patients with inadequate mitogen response or high background interferon-gamma levels 1, 6
Evolution of the Test
The original QuantiFERON-TB Gold has been succeeded by newer versions:
- QFT-Gold In-Tube (QFT-GIT) approved 2007 - uses pre-loaded tubes for easier specimen handling 7
- QFT-Gold Plus - adds TB2 tube to detect CD8+ T cell responses 8, 9
The newer versions maintain similar performance while improving operational convenience 7.