QuantiFERON-TB Gold Plus (QFT-Plus): Introduction, Principle, Procedure, Result Interpretation, Clinical Significance, and Keynotes
Introduction
Table of Contents
The QuantiFERON-TB Gold Plus (QFT-Plus) is a highly sensitive fourth-generation Interferon Gamma Release Assay (IGRA). It acts as an indirect, in vitro blood screening test used to identify infections caused by the Mycobacterium tuberculosis complex. This advanced laboratory assay serves as a modern alternative to the classic Tuberculin Skin Test (TST), delivering precise numerical tracking using only a single blood draw.
Principle
The core technology relies on evaluating cell-mediated immune responses:
T-Cell Recognition: Individuals infected with TB harbor memory T-lymphocytes that specifically recognize TB-associated proteins.
Antigen Stimulation: When whole blood is mixed with synthetic peptide cocktails mimicking ESAT-6 and CFP-10 bacterial proteins, these memory T-cells respond by releasing a signaling cytokine called Interferon-gamma (IFN-γ).
Dual T-Cell Targeting: Unlike older versions that only stimulated helper CD4+ T-cells, QFT-Plus targets both CD4+ and cytotoxic CD8+ T-cells, yielding a more robust diagnostic profile.
ELISA Detection: The precise quantity of secreted IFN-γ is subsequently measured using an Enzyme-Linked Immunosorbent Assay (ELISA).
Procedure
The standardized testing workflow consists of three major components:
Controlled Blood Collection: Exactly 1 mL of blood is collected into four dedicated, specialized tubes:
Nil Tube (Gray Cap): Negative control. Used to map the patient’s baseline internal IFN-γ levels.
TB1 Antigen Tube (Green Cap): Coated with long peptides designed to induce helper CD4+ T-cell responses.
TB2 Antigen Tube (Yellow Cap): Coated with a modified peptide cocktail optimized to prompt both CD4+ and CD8+ T-cell responses.
Mitogen Tube (Purple Cap): Positive control. Coated with a non-specific cell stimulant to ensure the blood sample contains viable, functioning immune cells.
Incubation: The four tubes must be shaken and moved to a 37°C incubator within 16 hours of collection, where they incubate for 16 to 24 hours.
Measurement: Tubes are centrifuged, plasma is harvested, and the exact concentration of IFN-γ is determined through ELISA testing.
Result-Interpretation
Qualitative diagnostic outcomes are determined mathematically by subtracting the baseline Nil value from the antigen readings:
Nil tube
0.10
IU/ml
TB1 Antigen tube
0.15
IU/ml
TB2 Antigen tube
0.19
IU/ml
Mitogen tube
4.68
IU/ml
TB1 Antigen tube minus Nil Tube
0.05
IU/ml
TB2 Antigen tube minus Nil Tube
0.09
IU/ml
Mitogen tube minus Nil tube
4.58
IU/ml
Final Qualitative Result
Negative
–
Positive Result: If the value of (TB1 minus Nil) OR (TB2 minus Nil) is ≥ 0.35 IU/ml AND at least 25% of the Nil value, a TB infection is highly likely.
Negative Result: If both calculations are < 0.35 IU/ml (or ≥ 0.35 but < 25% of Nil) AND the positive Mitogen control functions normally (≥ 0.50 IU/ml), a TB infection is NOT likely.
Indeterminate Result: If the baseline Nil tube is excessively high (>8.0 IU/ml), or if the Mitogen tube shows poor cellular activation (<0.50 IU/ml), the test is inconclusive. This usually highlights sample degradation or advanced patient immunosuppression.
Clinical Significance
No BCG Cross-Reactivity: The antigens used do not cross-react with the BCG vaccine or most environmental mycobacteria. This eliminates the false positives common with standard skin testing.
Latent vs. Active Status: The test checks for an immune memory response. It cannot differentiate between a dormant, non-infectious Latent TB Infection (LTBI) and symptomatic, contagious Active TB disease.
Keynotes
Comprehensive Diagnostic Integration: A positive test must always be combined with clinical risk profiles, physical examinations, chest radiographs, and sputum diagnostics.
Immune Window Period: False-negative results can occur if a patient is tested too soon after exposure. The CDC recommends re-testing after 8 to 10 weeks if suspicion remains high.
Immunocompromised Variations: Patients taking immunosuppressive medications or struggling with low T-cell counts may exhibit blunted immune reactions, leading to false negatives or indeterminate readings.
Introduction Organism: Gram-negative, encapsulated, facultative anaerobic, rod-shaped bacterium. Epidemiology: Major cause of healthcare-associated infections (HAIs)…