Defining 'sign' and 'symptom'

The terms 'sign' and 'symptom' have proven difficult to define and represent in a biomedical ontology. Medical professionals use 'sign' and 'symptom' to refer to medically relevant information about patients; however, they do not agree on the definitions. In particular, while medical professionals agree that there is an important distinction between signs and symptoms, they do not agree on the precise nature of this distinction. It is unsurprising then that attempts to provide ontological representations of these entities have repeatedly fallen short. As an added complication, a variety of entities-including material entities, qualities, and processes-may reasonably be understood as signs or symptoms. Thus, the ontological nature of a sign or symptom raises many questions about the meanings and proper use of these terms. We discuss specific challenges to defining 'sign' and 'symptom', identify essential features of these entities, explore the ontological implications of existing definitions, and propose our own definitions. We evaluate several competing ontological representations and present our proposed representation within the framework of the Ontology for General Medical Science. The proposed representation of sign and symptom is ontologically sound, provides precise definitions of each term, and enables users to easily create customized groups of signs and symptoms. Our experience highlights general issues about developing definitions in ontologies.