Methods for detecting estradiol and metabolites thereof in a sample are provided. A plurality of particles, each of which is coated with a capture agent having an affinity for estradiol, is combined with the sample to form a plurality of estradiol-particle complexes. The system also includes a transport arrangement for transporting the sample and/or particles to the sensor surface, and optionally a magnetic field inducing structure constructed and arranged to establish a magnetic field at and adjacent to the sensor surface. The resonant sensor produces a signal corresponding to an amount of estradiol-particle complexes that are bound to the sensor surface.