This paper presents EXEMED, a knowledge base created for the definition of executable rules that can be applied over Electronic Health Records (EHR) in order to measure the EHR compliance with respect to a specific clinical guideline. EXEMED is based on description logic and provides the concepts and roles necessary to express the sets of patients relevant for the definition of rules based on a clinical guideline. Its principle is to provide a mechanism to define the characteristics of patients that are pertinent for a specific clinical guideline through nine concepts, in such a way that they can be obtained from the whole universe of patients. The usefulness of EXEMED was validated applying it to a study case with three different clinical guidelines: Gastrointestinal Bleeding, Chronic Kidney Disease and Hipokalemia.