ig. 2 Decision points that determine the outcome of COVID-19 infection and requirements for a monocyte activation assay. A time and severity pattern has emerged for COVID-19 progression. The first days, first and second week timelines are turning points for resolution or deterioration. Epidemiologic precision for these milestones and precise parameters will be valuable for strategic sampling and patient stratification, sparing resources and patient material. With limited volumes of peripheral blood, a window on progression of the infection can be obtained by studying activation of the mononuclear phagocyte system, largely represented by monocytes in blood. The cells are biosynthetically active and relatively long lived, ideal to trace long term effects of comorbidities and infection. We highlight selected functional categories that should be addressed in monocytes and can be adapted to particular applications.