The invention concerns a diagnostic tape unit with a test tape that can be wound onto a spool which comprises a transport tape and a plurality of test elements mounted thereon, where the test elements have an analytical reagent layer, a carrier foil supporting the reagent layer and a piece of adhesive tape connecting the carrier foil with the transport tape, and where the front side of the reagent layer facing away from the carrier foil is designed for the application of a sample substance. According to the invention it is proposed that the test elements in combination with the light transmitting transport tape in each case form an optical multi-layer system for a rear-side reflection-photometric measurement of the reagent layer.