The present invention relates to systems and methods for detecting analyte molecules or particles in a fluid sample and in some cases, determining a measure of the concentration of the molecules or particles in the fluid sample. Methods of the present invention may comprise immobilizing a plurality of analyte molecules or particles to form a plurality of complexes, releasing at least a portion of some of the plurality of complexes, determining at least a portion of the plurality of complexes released, and determining a measure of the concentration of the analyte molecules or particles in a fluid sample.