Identifying eligible patients is one of the most critical parts of any clinical trial. The process of recruiting patients for the third phase of any clinical trial is usually done manually, informing relevant physicians or putting notes on bulletin boards. While most necessary information is already available in electronic hospital information systems, required data still has to be looked up individually. Most university hospitals make use of a dedicated communication server to distribute information from independent information systems, e.g. laboratory information systems, electronic health records, surgery planning systems. Thus, a theoretical model is developed to formally describe inclusion and exclusion criteria for each clinical trial using a fuzzy ternary logic expression. These expressions will then be used to process HL7 messages from a communication server in order to identify eligible patients.