The invention is related to an apparatus for extracorporeal treatment of blood with a dialyzer which is separated by a semipermeable membrane into a first and a second chamber, wherein the first chamber is disposed in a dialysis fluid pathway and the second chamber is connectable to the blood circulation of a patient by means of a blood inflow conduit and a blood outflow conduit, a feed for fresh dialysis fluid, a discharge for spent dialysis fluid, a means for estimation of a flow QBlood in the blood inflow conduit respectively the blood outflow conduit, a means for determining a flow QDialysate in the dialysis fluid pathway, and a measuring device disposed in the discharge for estimation of the absorption ADialysate of the spent dialysis fluid flowing through the discharge, wherein the measuring device has at least one radiation source and a detector system for detection of the intensity of the electromagnetic radiation. Therein provided are a memory, wherein a formula A Blood = ( A Dialysate · Q Dialysate Q Blood ) is stored for the absorption ABlood, and an arithmetic unit for estimation of the absorption ABlood in the blood inflow conduit respectively the blood outflow conduit using the absorption ADialysate, the flow QDialysate, and the flow QBlood.
