Apparatus and methods for the extracorporeal treatment of blood are described. The apparatus includes a dialyzer which is separated into a first and second chamber by a semipermeable membrane, wherein the first chamber is disposed in a dialysis fluid path and the second chamber can be connected to the blood circulation of a patient by way of a blood inflow conduit and a blood outflow conduit, a feed for fresh dialysis fluid, a discharge for spent dialysis fluid, a measuring device disposed within the discharge for determining the absorption of the spent dialysis fluid flowing through the discharge, wherein the measuring device has at least one radiation source for substantially monochromatic electromagnetic radiation, and a detector system for detecting the intensity of the electromagnetic radiation, wherein means are provided to compensate for changes that occur in the intensity of the electromagnetic radiation of the radiation source and/or the sensitivity of the detector system.