Blood tests are available that measure interferon-g release from T cells after stimulation with antigens largely specific to M. tuberculosis [such as early secreted antigen target (ESAT-6) and culture filtrate protein (CFP-10)]
The current commercially available tests are T-Spot TB and QuantiFERONs Gold In-Tube. Both tests are approved for the diagnosis of latent TB infection in HIV-negative individuals. Limited data exist regarding their performance in HIV infection, but studies suggest that interferon-g assays are more specific than TSTs, especially in BCG-vaccinated subjects. They appear to retain sensitivity more reliably at lower CD4 cell counts. However, the blood samples need processing within a limited time, and ‘indeterminate’ (i.e. uninterpretable) IGRA results are more common in HIV-infected subjects.
There are some differences between the two tests, although in general they are unaffected by previous BCG and/or infection with most other mycobacteria (an important exception in the United Kingdom being Mycobacterium kansasii).
Based on current data, we suggest that IGRAs rather than TSTs are used when screening HIV-positive individuals for latent TB infection. Where a patient is considered to have active TB, IGRA tests should not be used as the means by which the diagnosis is confirmed or refuted. If used, the result must be interpreted in light of the clinical picture, microbiological data and an understanding of the assay’s limitations in this population.