Therapeutic drug monitoring provides the attending physicians with detailed information on a patient’s individual serum level especially during long-term medication. Due to the fact that each patient tolerates drugs or their metabolites differently a medication adjustment can reduce the number and intensity of noticeable side-effects. In particular, psychotropic drugs can cause unpleasant side-effects that affect a patient’s life almost as much as the mental disease itself. The tricyclic antidepressants amitriptyline is commonly used for treatment of depressions and was selected for the development of an immunoassay using the direct optical sensor technique Reflectometric Interference Spectroscopy (RIfS). RIfS is a simple, robust and label-free method for direct monitoring of binding events on glass surfaces. Binding to the surface causes a shift of the interference spectrum by a change of the refractive index or physical thickness. This technique can be used for time-resolved observation of association and dissociation of amitriptyline (antigen) and a specific antibody using the binding inhibition test format. An amitriptyline derivative is immobilized on the sensor surface and a specific amount of antibodies can bind to the surface unless the binding is inhibited by free amitriptyline in a sample. No fluorescent label is needed making the whole assay less expensive than label-based methods. With this recently developed immunoassay amitriptyline concentrations in buffer (PBS) can easily be detected down to 500 ng/L.