Clinical decision support systems (CDSS) can significantly increase the quality of care while decreasing cost and effort. They are difficult to develop and most existing systems are proprietary, tightly integrated with specific electronic medical record (EMR) systems, and expensive to own. EGADSS is an open-source CDSS that has been developed as a standalone, standards-based, re-usable component to make decision-support available for any EMR. In order to realize this vision, the EGADSS team has had to develop an open interface for medical data exchange, which maximizes interoperability, simplicity and standardconformance. This paper reports on a solution to this challenge based on the HL7 Clinical Document Architecture (CDA) and the electronic medical summary standard. CDA-based medical summaries are used to encapsulate virtual medical records about patients that serve the DSS component as temporary databases. We show how these temporary databases can be queried from within Arden syntax-based guidelines in a standard query language. Moreover, we show how the CDA can be used to communicate CDSS alerts and recommendations back to the EMR. We report on our evaluation with a prototype implementation and compare it with alternative approaches.