The importance of decision support systems is highly acknowledged as a key strategy to improve medical safety and quality of care. A strong interoperability between the hospital Electronic Health Record (EHR) and the Clinical Decision Support System (CDSS) is the key to reach a most reliable decision support, that could aid in better diagnosis, reduce medication errors and improve practitioner performances. Interoperability is granted by the use of standards for data representation and for system intercommunication. A CDSS to support HAART (Highly active antiretroviral therapy) prescription in HIV (Human Immunodeficiency Virus) naive patients was developed within the Ligurian HIV Network. GLIF3 (GuideLine Interchange Format) standard was used to represent clinical guidelines in machine interpretable format. HSSP (Healthcare Services Specification Project) DSS (Decision Support System) standard was used to develop the CDSS web service that evaluates patient's data according to the rules emerging from the GLIF representation of the guidelines. Patient's data are extracted from hospital EHR, formatted into standard vMR (virtual Medical Record) documents and sent to the DSS web service to be evaluated. The results are displayed by a client application in an intuitive way to guide physician's decisions.