The invention relates to an analysis system for the photometric determination of an analyte in a body fluid, having an analysis device and a test carrier. The test carrier has an evaluation zone in which a photometrically detectable change takes place as the result of a reaction of a sample of the body fluid which contacts the test carrier with a reagent contained in the test carrier. The analysis device comprises an optical emitter for emitting light onto the evaluation zone, an optical receiver for receiving light from the evaluation zone and for generating a measurement signal corresponding to the received light, and a measurement and evaluation device with a measuring unit and an evaluating unit. The measurement signal is amplified and digitalized in the measuring unit. In the evaluating unit, the concentration value is determined from the digitalized measurement signal. In the evaluating unit, a control value of a control parameter is detected at one detection point during the processing after the calculation step. An error in the measurement and evaluation device is recognized if the deviation of the control value from the expected value exceeds a predefined threshold value.