Regional Medical Laboratory

QuantiFERON TB Gold Plus (QFT-Plus) is a 4th generation in vitro diagnostic test that uses a peptide cocktail containing ESAT-6 and CFP-10 proteins to stimulate cells, particularly lymphocytes.  Numerous studies have demonstrated that the ESAT-6 and CFP-10 protein antigens stimulate Interferon-γ (INF- γ) responses in T cells from individuals infected with Mycobacterium tuberculosis, but generally not from uninfected or BCG-vaccinated individuals. 

The QFT-Plus technology assesses cell-mediated responses through a quantitative measurement of Interferon-γ (IFN-γ). Detection of IFN-γ by an enzyme-linked immunosorbent assay (ELISA) is used to identify in vitro responses to these peptide antigens that are associated with M. tuberculosis (MTB) infection.  The QFT-Plus aids in the diagnosis of active tuberculosis and in individuals with a LTBI. The QFT-Plus, an IGRA, can only be considered as helpful but insufficient for diagnosing M. tuberculosis complex infection in sick patients; however, a positive result can support the diagnosis of tuberculosis disease or LTBI.  Also important to note is that some medical treatments (e.g.  immunosuppressive drugs, chemotherapy, etc.) or conditions (e.g. lymphopenia, HIV infection/AIDS, viral or intracellular infections, etc.) can impair immune functionality and potentially reduce the IFN-γ responses. 

In M. tuberculosis infections, CD4+ T cells play a critical role in immunological control through their secretion of the cytokines IFN-γ.  Evidence now supports a role for CD8+ T cells participating in the host defense to M. tuberculosis by producing IFN-γ and other soluble factors, which activate the macrophages to suppress growth of M. tuberculosis, kill infected cells, or directly lyse intracellular M. tuberculosis organisms.  IFN-γ producing MTB-specific CD8+ cells which have been detected in subjects with LTBI and with active TB.  Moreover, ESAT-6 and CFP-10 specific CD8+ T lymphocytes are described as being more frequently detected in subjects with active TB disease versus LTBI and may be associated with a recent MTB exposure or infection onset. 

QFT-Plus has two distinct TB antigen tubes: 

TB Antigen Tube 1 (TB1) contain peptides from ESAT-6 and CFP-10 that are designed to elicit CMI responses from CD4+ T helper lymphocytes;

TB Antigen Tube 2 (TB2) contain peptides from ESAT-6 and CFP-10 that are designed to elicit CMI responses from CD4+ T helper lymphocytes; and an additional set of peptides targeted for the induction of CMI responses from CD8+ cytotoxic T lymphocytes.  The results from tube 1 versus tube 2 may provide clues to distinguish a patient as to whether they have active tuberculosis or LTBI.