It is well known that human immunodeficiency virus (HIV) compromises the immune system of an infected individual, which in turn increases the risk of a new TB infection. However, not all of those infected with TB will display symptoms – known as latent TB infection (or LTBI). It is also known that HIV infection dramatically increases the risk of LTBI becoming active TB disease. Those co-infected with TB and HIV are 21 to 34 times more likely to develop active TB disease than people without HIV. The risk of developing active TB in individuals who are positive for HIV is increased many fold even when antiretroviral chemotherapy is given.
TB/HIV co-infection is a major challenge to healthcare providers, since the two conditions act together synergistically, each exacerbating the other to cause significant morbidity and mortality. TB is the leading cause of death of people living with HIV infection, and an estimated one quarter of TB deaths every year are HIV related.
Due to this significantly increased risk in individuals with HIV, an effective method of identifying TB infection is essential. Interferon gamma release assays (IGRAs) offer a new alternative to conventional methods of diagnosis. IGRAs measure interferon-gamma (IFN-γ) release after exposure to a specific antigen or mitogen in blood.
The QuantiFERON-TB Gold (QFT®) test specifically measures the cell-mediated immune response to specific TB antigens in whole blood. It relies on the principle that sensitized T-cells from a whole blood sample produce the cytokine IFN-γ when incubated with antigens specific for TB.
IFN-γ is then measured using a standard ELISA procedure. The accuracy of the QFT assay is unaffected by prior BCG vaccination and it is proven in immunocompromised patients.