The QuantiFERON-TB Gold test, often called QFT-Gold, is a blood test used to detect the presence of the bacterium that causes tuberculosis, Mycobacterium tuberculosis. This test falls into a category known as Interferon-Gamma Release Assays, or IGRAs. The fundamental purpose of the QFT-Gold test is to identify an individual’s immune response to the bacteria, providing information about a potential infection.
The Testing Procedure
The process for a QuantiFERON-TB Gold test begins with a standard blood draw from a vein, typically in the arm. The collected blood is distributed into a set of specialized tubes for the assay.
The inner surfaces of these tubes are pre-coated with antigens to provoke an immune response and control substances to ensure the test is working correctly. Once the blood is collected, the tubes are sent to a laboratory. They undergo a controlled incubation period before the final analysis is performed.
How the Test Works
After the blood is collected into the specialized tubes, the white blood cells are directly exposed to the tuberculosis antigens coating the tubes. These antigens are synthetic peptides that mimic proteins found in Mycobacterium tuberculosis, such as ESAT-6 and CFP-10.
If an individual has been previously infected with TB, their body will have produced specialized immune cells called T-lymphocytes that “remember” the bacterium. When these T-lymphocytes in the blood sample are re-exposed to the TB antigens in the tube, they respond by releasing a chemical messenger known as interferon-gamma (IFN-γ). The laboratory then measures the amount of IFN-γ produced in the sample to determine if the immune system shows a response to the TB antigens.
Interpreting the Results
A positive result indicates that a significant amount of interferon-gamma was released in response to the TB antigens. This signifies that the person’s immune system has had a previous encounter with the tuberculosis bacterium. A positive result confirms a TB infection, but it does not differentiate between latent TB infection (LTBI) and active TB disease. In LTBI, the bacteria remain in the body in a dormant state, while in active TB, the bacteria are multiplying and causing illness.
To determine if the disease is active, a positive test result necessitates further diagnostic evaluation. These follow-up procedures typically include a chest x-ray and a clinical assessment for symptoms of active TB. A negative result means that the test did not detect a meaningful immune response to the TB antigens. This suggests that a tuberculosis infection is unlikely.
Occasionally, a test may yield an indeterminate or borderline result. This outcome is inconclusive and can arise from several factors, such as an issue with the blood sample itself or a weakened immune system that prevents a clear response. In cases of an indeterminate result, a healthcare provider will often recommend that the test be repeated to obtain a clear conclusion.
Comparison with the Tuberculin Skin Test
The QuantiFERON-TB Gold test offers several distinct differences when compared to the older tuberculin skin test (TST), also known as the PPD test. A primary advantage is that the QFT-Gold test requires only one patient visit for a blood draw. The TST, in contrast, requires two visits: one for the injection of the testing substance and a second visit 48 to 72 hours later for a healthcare worker to visually inspect and measure the reaction on the skin.
A significant scientific advantage of the QFT-Gold test is that its accuracy is not affected by a prior Bacille Calmette-Guérin (BCG) vaccination. The BCG vaccine is commonly administered in many countries to prevent severe TB disease in children and can cause a false positive result on a TST. Because the QFT-Gold test uses specific antigens not present in the BCG vaccine, this cross-reactivity is avoided, leading to more reliable results for vaccinated individuals.
The results of the QFT-Gold test are obtained from a controlled laboratory analysis, making them objective and not subject to the variations of reader interpretation that can occur with a TST. While the QFT-Gold test is now widely used, some guidelines may still suggest a TST for certain populations, such as in children under the age of five.