Methods and systems for rapidly determining the level of platelet inhibition in whole blood, due to aspirin usage, with a single use arachidonic based assay device that can be stored at room temperature is provided. A lyophilized assay reagent that contains arachidonic acid at sufficient concentration to maximally activate platelets is utilized. An antioxidant within the same lyophilized assay reagent reduces the oxidation rate of arachidonic acid but does not interfere with platelet function. An oxygen absorber within the single use assay device packaging creates an inert environment within a short period of time after package is sealed. The assay device can have a housing with a plurality of channels and a common blood sample introduction port coupled to each of a channel of the plurality of channels. The assay device can also include a lyophilized assay reagent that contains arachidonic acid at sufficient concentration to maximally activate platelets.