Clinical guidelines systematically assist practitioners to provide appropriate health care in specific clinical circumstances. Today, a significant number of guidelines and protocols are lacking in quality. Indeed, ambiguity and incompleteness are likely anomalies in medical practice. Our objective is to find anomalies and to improve the quality of medical protocols using well-known mathematical formal techniques, such as Event B. In this paper, we use the Event B modelling language to capture guidelines for their validation. Our main contributions are: to apply mathematical formal techniques to evaluate real-life medical protocols for quality improvement; to derive verification proofs for the protocol and properties according to medical experts; and to publicize the potential of this approach. An assessment of the proposed approach is given through a case study, relative to a real-life reference protocol (ECG interpretation), which covers a wide variety of protocol characteristics related to several heart diseases. We formalize the reference protocol, verify a set of interesting properties of the protocol and finally determine anomalies.