Microfluidic systems are becoming common in the development of point-of-care (PoC) diagnostic systems where various methods are used to efficiently separate blood cells from whole blood. The goal of this research is to develop a passive plasma separator that can easily be integrated into an entire blood-based diagnosis microfluidic platform. In the present work, a one-step process of creating a cell-free region in the flow without the plasma being actively extracted from the whole blood is discussed. Centrifugal force together with a backward facing step is utilized to create a blood cell-free zone. Sensors (typically less than 10 micrometers in size) are proposed to be placed in the cell-free zones for biomarker detection. A detailed numerical study for the design of the microfluidic platform is reported. The two-phase nature of the blood is modeled using a discrete element method (DEM) where blood cells are modeled as spherical constituents with the inclusion of inter-particular interactions. The sizes of the cell-free zones in the microfluidic system are measured for various geometric and flow conditions. An expansion chamber with a larger aspect ratio together with a low Reynold number flow entering it is observed to create a larger cell-free zone.