An assay cartridge for mixing a test sample and reagent for diagnostic purposes, and method of using the same. The assay cartridge includes a housing, an injection port for receiving the test sample (whole blood), a filtration membrane for separating plasma from the sample, a section for receipt of reagent, and one or more sets of valves provided in the housing. Each valve is configured for movement between an open position and a closed position. Metering chambers are provided in the housing for receiving separated plasma and the reagent therein. A port is fluidly connected to each metering chamber and uses to draw air from or deliver pressurized fluid into the housing. Mixing channels are provided in the housing and selectively connected to the metering chambers for mixing the separated plasma and reagent into a substantially homogeneous mixture. The homogeneous mixture is delivered out of the housing through output ports.