The present invention relates to a device (1) and a corresponding method for determining residual blood in a dialyzer (2) comprising a holding device (6) configured to accommodate a dialyzer (2), an image capturing device (7) configured to capture at least one image of a dialyzer (2) accommodated by the holding device (6), in particular a filter (4) located in the dialyzer (2), and generate corresponding image data having a plurality of image points, each of which assigned at least one respective intensity value, in particular a color value, and a control device (9) configured to determine residual blood information characterizing the presence and/or content of residual blood in the dialyzer (2) on the basis of a statistical frequency of at least a portion of the intensity values in the at least one image. The invention further relates to a dialysis system having such a device (1).