Repetitive laboratory testing has become a well-recognized problem in the practice of medicine, especially in the hospital inpatient setting, since it increases costs and causes patient discomfort. Among the interventions proposed to reduce unnecessary testing, Clinical Decision Support Systems (CDSS) have been shown to be effective. We present the project of a CDSS recommending professionals in real time regarding the appropriateness for repeating laboratory exams, embedded in a Computerized Physician Order Entry at the Azienda Ospedaliero-Universitaria and Azienda Unita Sanitaria Locale of Ferrara, Italy. Test-specific time intervals within which test repetition is redundant are encoded in formal rules, which are applied against the laboratory results done in the past for the patient. The rules-set implemented concerns: clinical chemistry, hematology, coagulation, infectious diseases serology, microbiology, inflammation, cardiac and tumor markers, hormones, autoimmunity, allergology, molecular biology and drug monitoring testing.