Inhibitors of the PD-1 pathway have revolutionized oncology, however, only a fraction of patients will respond and identifying that fraction of patients either before treatment or even early in the course of treatment remains difficult. We are exploring whether improvements in sensitivity with Simoa will enable peripheral blood-based monitoring of the anti-tumor immune response for patients on checkpoint inhibitor therapy. So far we have shown that four cytokines, especially at very low sub pg/ml concentrations, can be reliably measured by Simoa from clinical plasma specimens (Cohen and Keegan, et al. Clin Biochem. 2019). Now we are characterizing whether successful PD-1i-mediated anti-tumor response is accompanied by recurrent changes in soluble inflammatory molecules in the peripheral blood.