Interferon Gamma Release Assays in active Tuberculosis: new medical insights

Since first presentation, Interferon γ Release Assays (IGRAs) have had basic and wide application to LTBI, in accordance with international consensus and CDC recommendations, leaving their use in active TB to the field of study and research.We reviewed the results of 633 patients investigated from 2004 to 2008 targeting active TB, with the objective to highlight immunological data supporting test performances.We evaluated Quantiferon TB Gold (1st generation IGRA kit) in association to Culture (MGIT 960 and Lowenstein Jensen) and PCR (Probetec-ET) having the positivity of culture plus clinical diagnosis as the standard true value to compare. QTB Gold was studied in 69 TB positive patients (42 pulmonary and 27 extra-pulmonary), with Sensitivity, Specificity, PPV and NPV average to 61.8%, 94.5%, 54.3% and 95.9% respectively, after indeterminate results discharging. Significant statistical differences didn’t emerge between pulmonary and extra-pulmonary infections (CI 95%).The overall indeterminate ratio arose up to 20.3% in patients with active TB vs 2.7% of global population (p<0.001). In 22% of patients with active pulmonary disease, IGRA conversed to positivity after 15 days in replicated tests, in spite of current treatment. 4 patients, with pulmonary TB and Quantiferon persistent negativities, underwent 18 months follow-up as not respondent although SIRE phenotypic susceptibilities and enough DOT compliance. Molecular DST documented hetero resistance for rpoB (MUT 1, MUT 3 plus wild lines) and katG (MUT 1 plus wild) in association to lack of inhA wild lines (Genotype MTBDR plus, Hain Lifescience). These reports suggest a mutational relationship between Rv3874 – 3875 cassette, encoding ESAT-6 / CFP-10, and rpoB, katG, inhA genes plausibly implying weak or absent selective clonal Th 1 activation to IGRA antigens. Our data seem to point out: 1) positive results are able to match true active TB in less than 50% of patients; 2) negative results could leave undiagnosed or misdiagnosed more than 30% of active TB; 3) the higher level of indeterminate results and the conversing positivity in replicated IGRA tests might find their root either in Mycobacteria genome and microbiome variability or in the lymphocyte kinetic and APCs/DCs inhibition due to inflammatory engage, present in human active TB and already focused in animal model, with IL-10 key role like in T-Cell exhaustion phenotype.