Can a previously treated tuberculosis (TB) patient test positive on a QuantiFERON (Interferon-Gamma Release Assay) test?

From the Guidelines

Yes, you can get a positive QuantiFERON-TB Gold test result even after successful treatment for tuberculosis (TB). The QuantiFERON test measures the immune response to TB antigens and detects whether a person has been infected with Mycobacterium tuberculosis at some point in their life. Unlike the tuberculin skin test, the QuantiFERON test does not typically revert to negative after treatment. This is because the test detects memory T-cells that remain in the body long after the infection has been cleared. These memory cells continue to recognize TB antigens and produce interferon-gamma when exposed to them during the test. Therefore, the QuantiFERON test cannot distinguish between active TB, latent TB infection, or previously treated TB. For this reason, the test is not recommended for monitoring treatment response or determining cure. If you have a history of treated TB and need to be evaluated for possible new infection, your healthcare provider should rely on clinical symptoms, chest X-rays, and other diagnostic tests rather than the QuantiFERON result alone 1.

Some key points to consider:

• The QuantiFERON test can remain positive after treatment of latent TB, as stated in the guidelines for the management and treatment of psoriasis with biologics 1.
• Yearly testing for latent TB should be done in patients at high risk, and the QuantiFERON Gold test can be used for this purpose, but caution should be exercised when interpreting the results 1.
• The test is not recommended for monitoring treatment response or determining cure, and clinical symptoms, chest X-rays, and other diagnostic tests should be used instead 1.

In terms of morbidity, mortality, and quality of life, it is essential to consider the potential risks and benefits of using the QuantiFERON test in patients with previously treated TB. While the test can provide valuable information about the presence of TB infection, it is crucial to interpret the results in the context of the patient's medical history and clinical symptoms. By doing so, healthcare providers can make informed decisions about the best course of treatment and minimize the risk of adverse outcomes. 1.

From the Research

Positive Quantiferon Results for Previously Treated TB

• A study published in 2010 2 found that the QuantiFERON-TB Gold In-tube (QFT-TB) test was positive in 71.4% (20/28) of individuals who had been previously treated for TB.
• The same study also found that after preventive therapy, 35/40 (87.5%) and 22/26 (84.6%) were still QFT-TB positive after three and 15 months, respectively.
• This suggests that a positive Quantiferon result can occur in individuals who have been previously treated for TB, and that the test may not be useful for monitoring the effect of preventive therapy.

Factors Associated with Positive QFT-TB Results

• The 2010 study 2 found that origin from a TB endemic country, recent stay in a TB endemic country, duration of TB exposure, and previous TB disease were all independently associated with a positive QFT-TB test.
• These factors may contribute to the likelihood of a positive Quantiferon result in individuals who have been previously treated for TB.

Limitations of QFT-TB Test

• The QFT-TB test may not be useful for monitoring the effect of preventive therapy, as most patients remained QFT-TB positive after therapy 2.
• The test may have limited benefits in populations with low risk of TB, and may not be necessary in addition to other screening methods such as chest X-rays 3.