The QuantiFERON-TB Gold Test (QFT) is a modern blood test that detects infection with Mycobacterium tuberculosis, the bacteria responsible for tuberculosis (TB). This interferon-gamma release assay (IGRA) identifies a person’s immune response to TB bacteria. It aids in diagnosing both latent TB infection and active TB disease, offering an alternative to traditional screening methods.
Why the Test is Used
The QFT is recommended for populations at increased risk of TB infection. This includes individuals in close contact with someone diagnosed with active TB, and those with compromised immune systems, such as people living with HIV or organ transplant recipients.
Healthcare workers and immigrants from countries with high TB prevalence are also screened using the QFT. Its convenience and accuracy make it an effective tool for identifying and managing TB infections.
How the Test Works and is Performed
The Quantiferon-TB Gold Test (IGRA) detects the body’s immune response to specific TB antigens. When infected with Mycobacterium tuberculosis, immune cells (T-cells) develop a memory of the bacteria.
For the test, a blood sample is collected into specialized tubes containing synthetic peptides representing TB antigens like ESAT-6 and CFP-10. If TB-specific T-cells are present, they recognize these antigens and release interferon-gamma (IFN-γ). The amount of IFN-γ released is measured using an enzyme-linked immunosorbent assay (ELISA). A higher IFN-γ level indicates an immune response to the TB bacteria, suggesting infection.
Interpreting Your Results
Quantiferon-TB Gold Test results are reported as positive, negative, or indeterminate.
A positive result indicates Mycobacterium tuberculosis infection is likely. This means TB bacteria are present, but it does not differentiate between latent TB infection and active TB disease. Further medical evaluation, such as a chest X-ray, is necessary to rule out active disease.
A negative result suggests TB infection was not detected. This means the immune system did not respond to the TB antigens. However, a negative result does not completely exclude infection, especially with recent exposure or weakened immune systems.
An indeterminate result means the test could not provide a definitive outcome. This can occur due to a weak immune response or improper blood sample handling. In such cases, retesting or alternative diagnostic evaluations may be recommended.
QFT vs. Other TB Tests
The QuantiFERON-TB Gold Test offers several advantages over the traditional Tuberculin Skin Test (TST), also known as the PPD test. The QFT requires only a single patient visit for blood collection, eliminating the need for a follow-up appointment to read a skin reaction. Unlike the TST, the QFT is not affected by prior Bacille Calmette-Guérin (BCG) vaccination, which can cause false-positive results with the skin test.
The QFT provides objective laboratory results, reducing subjective interpretation errors common with TST readings. While the QFT cannot distinguish between latent and active TB infection—a limitation it shares with TST—it offers a more precise measurement of the immune response. However, the QFT can be more expensive than the TST and requires a specialized laboratory for processing.