Interferon Gamma as a Biomarker for Diagnosis of Tuberculosis: Current Status

After entrance of Mycobacterium tuberculosis (MTB) bacilli inside the host, they leave foot prints in the form of antibodies and sensitized T cells, irrespective of the location (extra-pulmonary or pulmonary) of MTB inside the host. Thus, analysing antibodies and/or sensitized T cells inside the host could help in detection of MTB infection and thereby in confirming clinical diagnosis of tuberculosis. Over the years, measuring cell mediated immunity by tuberculin skin test employing purified protein derivative (PPD) has remained in practice for detection of MTB infection. However, such PPD involving assays were observed to be non-specific. During nineteen nineties, efforts were made towards developing interferon gamma (IFN-γ) based M.tuberculosis specific assays. Now it is know that both TST and IFN-γ based assays have advantages and disadvantages; and none of these tests suits all conditions. IFN-γ based assays eliminate the need for revisit of the subject while TST testing causes boosting and thereby creates interpretational problem in investigations requiring serial testing for monitoring. Under such situations adopting IFN-γ based assays could be more appropriate. Despite being highly sensitive and specific to MTB infection, IFN-γ based assay fails to differentiate between active and latent tuberculosis infection (LTBI). Hence, efforts towards making IFN-γ based assays useful to diagnose active tuberculosis are urgently required.