Search Terms" Emergency medical services, lights and siren, ranking, differentiating patients, use of priority medical dispatch, high-risk patients, low-risk patients, Annals of Emergency Medicine, 1990, Vol. 19 No. 4, Clawson's priority medical dispatch, priorities, study, indications, requests for ambulance Annals of Emergency Medicine, April 1990, Vol. 19, No. 4 G. Kallsen, M.D. Nabors/ Department of Health, Fresno County; Department of Emergency Medicine, Valley Medical Center, Fresno, California; Department of Family and Community Medicine, University of California, San Francisco; Department of Health, Fresno County, Fresno, California The Use of Priority Medical Dispatch to Distinguish Between High- and Low-Risk Patients Objective: Requests for ambulances are commonly prioritized with little published evidence validating the safety and efficacy of prioritization techniques. This study was to determine if variation of Clawson's priority medical dispatch ranks patients with prehospital cardiac arrest and other critical conditions into higher priorities than patients with more routine problems. Design: This is a retrospective analysis of centralized dispatch data. Setting: The study of emergency medical services [EMS] system serves a population of 600,000 and an area of 6,004 square miles in central California. All EMS calls are dispatched or tracked by one communication center using medical protocols and certified dispatchers. Participants: All 52,020 request for EMS dispatch in Fresno County in 1988 were analyzed. Exclusions of scheduled requests, transfers, physician or nurse requests, and nontransports resulted in 31,026 dispatches reported. Interventions: Dispatches were ranked as life threatening (priority 1), emergency (priority 2), or other (priority 3). Outcome was ranked as prehospital arrest for patients requiring prehospital CPR, critical condition for patients requiring lights and sirens to hospital, and routine for others. The indications for lights and sirens to hospital are standardized with written guidelines. Results: Priority 1 detected arrests with a sensitivity of .90 and specificity of .50. Priority 1 patients suffered prehospital arrest and critical condition more often than priority 2 patients, who suffered prehospital arrest and critical condition more often that priority 3 patients [P < .01 by two-tailed analysis of proportions]. Conclusion: This version of the Clawson model of priority medical dispatch successfully differentiates patients who suffer prehospital cardiac arrest or critical condition from less critical patients.