The invention relates to a detector for measuring scattered light in liquids having a housing, a transparent, flexible tube for transporting liquid through the housing, a light emitter, and a light detector. Two parallel surfaces are formed in the housing, between which the tube is arranged such that two opposing tube walls are formed in a planar parallel manner. The light emitter is arranged in such a way that the optical axis thereof is perpendicular to the parallel surfaces of the first tube wall, and the light detector is adjacent to the light emitter, the optical axes of the light emitter and light detector forming an angle smaller than 90°. The invention also relates to a method for detecting the presence of blood and for the quantitative determination of biological marker substances, especially bilirubin, in solution, and to a device for treating blood containing the detector.