This paper describes the evaluation of our current frame-based ontology in Intensive Care. Our approach to the evaluation consists of formalizing the ontology. The motivation for formalization is twofold: a. The formalization process itself can shed light on the implicit ambiguities in the ontology and b. The result of this process is a representation that facilitates automatic inference. The evaluation has revealed various ambiguities in the current ontology, and has clarified the path for migration towards a formal representation supporting automatic inference.