A diffusion immunoassay (DIA) for determining the presence and concentration of analyte particles by detecting the diffusion front. A hydrogel containing immobilized binding particles is placed in contact with a carrier fluid containing analyte particles, which analyte particles diffuse into the hydrogel and bind with the immobilized binding particles. A detection device detects the position of the diffusion front formed in the hydrogel to determine the presence and concentration of the analyte particles which have diffused into the hydrogel.