Methods for therapeutic drug monitoring are provided. A plurality of particles, each of which is coated with a capture agent capable of binding a therapeutic drug of choice is combined with the sample to form a plurality of therapeutic drug-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 therapeutic drug-particle complexes that are bound to the sensor surface.