The QuantiFERON-TB Gold Plus test, often called the TB Gold test, is a modern blood test known as an interferon-gamma release assay (IGRA). It is used to determine if an individual has been infected with Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB). The test is designed for use as part of a complete medical evaluation that may include assessment of risk factors and other diagnostic procedures.
When the TB Gold Test Is Used
One of the most common applications is in occupational health, particularly for individuals working in environments with a higher risk of transmission. This includes healthcare workers, employees in long-term care facilities, and school personnel who may be in close contact with vulnerable populations. A baseline test is often performed upon hiring to establish an individual’s status.
Screening is also recommended for individuals who have had known contact with someone who has active, contagious TB disease. Another use is for screening people with compromised immune systems. This group includes individuals with HIV, patients preparing to start medications that suppress the immune system for conditions like rheumatoid arthritis or Crohn’s disease, and those who are candidates for organ transplantation. Certain international travel or immigration processes may also require proof of TB screening.
Comparison to the TB Skin Test
For many years, the primary method for TB screening was the tuberculin skin test (TST), also known as the Mantoux test. The most noticeable difference for the patient is the procedure itself. The TB Gold test requires a single blood draw from a vein, with no need for a follow-up appointment to obtain the result. In contrast, the TST involves injecting a small amount of a substance called tuberculin into the skin of the forearm and requires the patient to return 48 to 72 hours later so a healthcare worker can visually inspect and measure the reaction.
Another advantage is objectivity. The TB Gold test is an in-vitro laboratory test that produces a quantitative, numerical result by measuring a specific immune response. This removes the subjective element of the TST, where the result can be influenced by the skill and interpretation of the person reading the skin reaction. The TST reading relies on measuring the size of the induration, or hardened swelling, which can be inconsistent.
The accuracy of the TB Gold test is not affected by a prior Bacillus Calmette-Guérin (BCG) vaccination. The BCG vaccine is widely administered in many countries to prevent severe TB in children and can cause a false-positive reaction to the TST. The TB Gold test uses antigens that are specific to the M. tuberculosis bacteria and are not found in the BCG vaccine strain, making it a more reliable test for individuals who have been vaccinated.
The Testing Procedure and Mechanism
A healthcare professional collects a sample of whole blood by drawing it from a vein in the arm. The blood is collected into specialized tubes that already contain specific antigens and controls necessary for the assay.
Once in the lab, the test works by assessing the response of the patient’s immune cells. The blood sample is incubated, allowing the T-lymphocytes (a type of white blood cell) to come into contact with the TB-specific antigens coated inside the collection tubes. If the patient’s immune system has been previously exposed to M. tuberculosis, their T-cells will recognize these antigens. This recognition triggers a defensive response, causing the T-cells to release a signaling protein called interferon-gamma (IFN-γ). The test measures the amount of IFN-γ produced, and a significant level of this protein indicates a positive reaction.
Interpreting Test Results
The results of a TB Gold test are reported as positive, negative, or indeterminate. A positive result indicates that the person’s immune system has been exposed to and infected with TB bacteria. A positive test does not distinguish between latent TB infection (LTBI) and active TB disease. In most cases, a positive result points to LTBI, a condition where the bacteria remain dormant in the body without causing illness or being contagious. Following a positive result, further medical evaluation, including a chest x-ray and a clinical assessment, is required to rule out active TB.
A negative result means that an infection with TB bacteria is not likely. This result is considered conclusive, especially for individuals with healthy immune systems. No further testing for TB is needed unless new exposures occur.
Occasionally, a test may yield an indeterminate or borderline result. Indeterminate results can happen for several reasons, such as a low level of immune response in people with weakened immune systems, or technical issues with the blood sample itself. In these instances, the physician will assess the situation and may recommend that the test be repeated.