The present invention relates to a magnetoresistive sensing device, a system and a method for determining a density of magnetic particles in a fluid. The magnetoresistive sensing device has a substrate (1) with a layer structure (2) for supporting a fluid (3). The layer structure has a first surface area (4) in a first level and a second surface area (5) in another second level and a magnetoresistive element (6) for detecting the magnetic field of at least one magnetic particle (7) in the fluid, the magnetoresistive element being positioned near a transition (8) between the first and second surface area and facing at least one of the surface areas. A corresponding system and method is described as well.