Small molecule analytes (less than 1000 Daltons) in a fluid sample are detected using a competitive assay in a magnetic biosensor. The fluid sample is added to a biosensor detection chamber together with detection probes and magnetic tags which bind to the detection probes. The magnetic biosensor is functionalized with a capture probe that shares an epitope with the analytes, and the detection probe is capable of binding the epitope shared by the analytes and the capture probe, so that the presence of the analyte prevents detection probes (and magnetic tags) from binding to the biosensor. By measuring the binding of the magnetic tags to the magnetic biosensor, an amount of analytes in the solution is determined.