Saliva is an underused fluid with considerable promise for biomedical testing. Its potential is particularly great for monitoring small-molecule analytes since these are often present in saliva at concentrations that correlate well with their free levels in blood. We describe the development of a prototype diagnostic device for the rapid detection of the antiepileptic drug (AED) phenytoin in saliva. The multicomponent system includes a hand-portable surface plasmon resonance (SPR) imaging instrument and a disposable microfluidic assay card.