What is TB blood testing? Recently, scientists have examined measuring Interferon Gamma in the blood as an alternative to tuberculin skin testing (TST). These tests measure Interferon Gamma that is released from sensitized lymphocytes (a type of white blood cell) from whole blood that was incubated overnight with certain antigens that are specific for Mycobacterium tuberculosis, the causative germ for tuberculosis.
As with the TST, positive blood testing cannot differentiate infection associated with TB disease from latent infection (LTBI). Likewise, negative results should not be used alone to exclude M. tuberculosis infection in persons with symptoms or signs suggestive of TB disease.
How does the predictive value the blood test compare to TST? Although there have been studies confirming the increased future risk of active TB in individuals with positive TST, the same was not true for those with a positive Interferon Gamma Release Assay (IGRA) result. In 2008, a study from Germany demonstrated that a positive IGRA result is predictive of future active TB risk (Diel R, Loddenkemper R, Meywald-Walter K, Niemann S, Nienhaus A. Predictive value of a whole-blood IFN-assay for the development of active tuberculosis disease after recent infection with Mycobacterium tuberculosis. Am J Respir Crit Care Med 2008;177:1164-1170). Moreover, IGRA was at least as sensitive and was more specific compared to traditional TST. Negative IGRA results do not require confirmation, but results can be confirmed with either a repeat IGRA or TST if the accuracy of the initial test is in question.With any of the testing methods, persons who have a negative test result can still have LTBI. Individuals with a negative IGRA result but who are likely to have LTBI and who are at greater risk for severe illness or poor outcomes if TB disease occurs might need treatment or closer monitoring for disease.
What are the currently approved IGRA tests? Currently approved (by the FDA) IGRA tests in the US are QuantiFERON TB Gold, QuantiFERON TB Gold In-Tube, and T-Spot.TB. The current CDC guidelines allow for the use of IGRA in all circumstances in place of PPD skin testing.
What are the advantages of IGRA testing?
- Single patient visit (no need to return in 48-72 hours)
- No anamnestic (“boosting”) response
- More specific (better able to separate those who might have had a false-positive PPD from prior BCG vaccination or many non-tuberculous mycobacterial infections)
- Results available in 24 hours
- No reader bias or error
What are the disadvantages of IGRA testing?
- More costly
- May be less available in certain locales
- False positive can occur in individuals with certain non-tuberculous mycobacterial infections
- Collection/Transporting/Laboratory errors
- Performance in immunocompromised individuals and children not completely determined
- Blood must be tested within 8-12 hours
Additional Information: Due to insufficient available information, IGRA is not recommended for screening of children under 17 years, pregnant women, or for those with diseases that increase the risk for progression of LTBI to active TB (e.g., HIV/AIDS). There is no reason to follow a positive IGRA with PPD testing. It is important to note that active TB cases can have a negative IGRA and a positive IGRA cannot differentiate between LTBI and active disease.
Where can I learn more?