A self-metering reaction device has a sample reservoir, configured to accept a varying amount of fluid; a metering reservoir, configured to be a subportion of the sample reservoir and to hold a reaction amount of the fluid; a reaction chamber fluidly connected to the metering reservoir; and a plunger comprising a tip configured to make a seal with the metering reservoir so that the reaction amount of the fluid is sealed within the metering reservoir when the plunger is in contact with the metering reservoir. The plunger can be configured to plunge the sealed reaction amount of the fluid from the metering reservoir into the reaction chamber.