A device and a process for testing a sample liquid wherein the sample liquid flows laminarly by capillary forces over a flat side of a channel to completely filling a reaction area which has a soluble and/or reacting reagent on the flat side and defines a reaction volume of the sample liquid, the reaction volume being temporarily stopped in the reaction area for dissolving or reacting at least 90% of the reagent in the defined reaction volume of the sample liquid, and at least 90% of the dissolved reagent or a reaction product, after stopping, flows together with the reaction volume into the test area which is formed by the channel downstream of the reaction area, the sample liquid flowing with an at least essentially straight liquid front and/or at least essentially without a change in the flow cross section from the reaction area into the test area.