From the Guidelines
The QuantiFERON-TB Gold test is the preferred method for testing for tuberculosis (TB) in patients with prior TB vaccination (BCG) due to its high specificity and ability to distinguish between BCG vaccination and actual TB infection. This interferon-gamma release assay (IGRA) specifically detects immune response to Mycobacterium tuberculosis antigens that are not present in the BCG vaccine, making it more specific than the traditional tuberculin skin test (TST) 1. The test requires a blood sample and results are typically available within 24-48 hours. Unlike the TST, the QuantiFERON-TB Gold test is not affected by prior BCG vaccination, which reduces false-positive results in vaccinated individuals. This is particularly important because BCG vaccination is common in many countries outside the United States. The test works by measuring the immune system's response to TB proteins; if the person has been infected with TB (even latently), their T-cells will recognize these proteins and release interferon-gamma.
Some key points to consider when using the QuantiFERON-TB Gold test include:
- It is preferred over the TST in patients with a history of BCG vaccination 1
- It is not affected by prior BCG vaccination, reducing false-positive results 1
- It requires a blood sample and results are typically available within 24-48 hours
- No special preparation is needed for patients undergoing this test, and a single visit is sufficient as no return visit is required to read results as with the TST
- The test is highly specific and can distinguish between BCG vaccination and actual TB infection, making it a valuable tool in diagnosing TB in patients with prior BCG vaccination 1.
From the Research
Appropriateness of QuantiFERON Gold Test for TB in Patients with Prior TB Vaccination
- The QuantiFERON Gold test is an interferon-gamma release assay (IGRA) that measures the immune response to tuberculosis (TB) antigens 2, 3, 4, 5, 6.
- IGRAs, including the QuantiFERON Gold test, are not affected by prior BCG vaccination, making them useful for detecting latent TB infection in patients with a history of vaccination 2, 3, 5, 6.
- The specificity of the QuantiFERON Gold test is higher than the tuberculin skin test (TST) in BCG-vaccinated populations, particularly if BCG is received after infancy 3, 6.
- The QuantiFERON Gold test has a sensitivity of 73.9% and specificity of 85.7% for detecting latent TB infection in patients with rheumatic disease 2.
- The test is useful for detecting latent TB infection in patients treated with corticosteroids due to its lack of correlation with corticosteroid therapy 2.
Comparison with Other Tests
- The QuantiFERON Gold test has been compared to the TST and other IGRAs, such as the T-SPOT.TB test, and has shown good agreement and correlation with these tests 2, 3, 4.
- The QuantiFERON Gold Plus assay, a newer version of the QuantiFERON Gold test, has been shown to have good agreement with the QuantiFERON Gold In-Tube assay and is potentially advantageous for patients with active TB, recent TB, or specific immune conditions involving CD8+ T-cell responses 4.
Limitations and Clinical Applications
- IGRAs, including the QuantiFERON Gold test, have limitations in serial testing due to spontaneous variability and lack of a validated definition of conversion and reversion 5, 6.
- The QuantiFERON Gold test is not sufficient to exclude or confirm active TB disease, and microbiological confirmation by culture of MTB is still the gold standard for diagnosis of active TB disease 5.