IFN-γ is a proinflammatory cytokine and a key regulator of the host immune system. In addition to host defense against pathogens, IFN-γ also contributes to autoimmune pathology by activating cytotoxic T cells and inducing target cell apoptosis. It is well established that IFN-γ plays an important role in regulating the progression of many chronic infections and autoimmune diseases. Therefore, IFN-γ is explored as a biomarker to be used in various stages of drug development, including translational studies or to monitor treatment responses in clinical trials. However, the protein expression level of IFN-γ in patient`s blood plasma or serum is usually very low, making it difficult to use IFN-γ as a biomarker to monitor disease progression. Therefore, we adopted the high sensitivity Single Molecule Counting (SMC™) technology to measure IFN-γ in the patient`s plasma/serum samples. Our results show that the IFN-γ measured by SMCTM technology correlated with disease severity and the expression of IFN-γ-induced chemokines, and thus IFN-γ could potentially be used as a disease biomarker.