A method and a device for differentiation of substances in a body fluid, such as blood, plasma or used peritoneal dialysis fluid, for example for hemodialysis. The device has a compartment having several inlets for entering a body fluid, a transition fluid and a diffusion fluid for flowing parallel with each other in laminar flow layers with substantially equal flow velocities. The transition fluid layer is interposed between the body fluid layer and the diffusion fluid layer. The compartment further has a first outlet for removing the body fluid and the transition fluid and a second outlet for removing the diffusion fluid. Pumps are arranged for controlling the flow velocities so that a marker substance, such as albumin, will not diffuse from the body fluid layer across the transition layer, during the passage of the body fluid from the inlet to the outlet of the compartment.