The present disclosure describes methods, devices, reagents, and kits for the detection of one or more target molecules that may be present in a test sample. In one embodiment, a test sample is contacted with an aptamer that includes a tag and has a specific affinity for a target molecule. An aptamer affinity complex that includes an aptamer bound to its target molecule is allowed to form. If the test sample contains the target molecule, an aptamer affinity complex will generally form in the test sample. The aptamer affinity complex is optionally converted to an aptamer covalent complex that includes an aptamer covalently bound to its target molecule. The aptamer affinity complex (or optional aptamer covalent complex) can then be detected and/or quantified using any of a variety of methods known to one skilled in the art, including using a solid support, using mass spectrometry, and using quantitative polymerase chain reaction (Q-PCR).