The present invention relates to a method and a device for determining a concentration of a biological active substance in a sample by the means of an enzyme-linked immunosorbent assay (ELISA). The device comprises a solid support within a tubing for binding an immunosorbent and an inlet for fluids, a detector for detecting radiation due to an activity in the tubing, wherein the tubing is arranged inside a microchip extending substantially in one plane, for conducting the fluids along the plane of the microchip. The tubing forms a reaction cell having a large detection area. The reaction cell of the microchip is arranged perpendicular to the detector. The method comprises the steps of introducing the fluids into the tubing, conducting the fluids through the tubing forming a reaction cell, in which reaction cell the radiation emitting activity takes place, and detecting the light emitted from the reaction cell substantially perpendicular to the plane.