A substance or reactant intended to be used together with a parent IVD to perform a specific function in an assay that is used for the qualitative and/or quantitative detection of interferon gamma (IFN gamma) released in response to T-cell activation resulting from stimulation by one or multiple specific antigens or mitogens, including (but not limited to) B Borrelia species, Epstein-Barr virus (EBV), Cytomegalovirus (CMV), BK virus, severe acute respiratory syndrome-associated coronavirus 2 (SARS-CoV-2), and/or Mycobacterium species.