The Estimated Prevalence of Multiple Record Encounters in the National Emergency Medical Services Information System.
Prehospital emergency care · 2026-08-18 · Observational study
Abstract
OBJECTIVES: The National Emergency Medical Services Information System (NEMSIS) is a large repository of de-identified Emergency Medical Services (EMS) data supported by the National Highway Traffic Safety Administration and is a cornerstone of large database EMS research. A known concern with large EMS datasets like NEMSIS is that multiple records may exist for a single episode of patient care. However, the extent of potentially related records is not well described. We evaluated potential EMS episodes of care contained within NEMSIS. METHODS: We used an observational study design and conducted a retrospective analysis of the 2023 NEMSIS dataset. We identified hierarchical clusters of likely related episodes of care for the same patient using tiered criteria of temporal (dispatch call times of 30 s to 2 min) and Census Division, EMS agency, urbanicity, patient demographics, dispatch characteristics, clinical impression, and location type. We evaluated all possible combinations of these criteria and reported the percentage of episodes of care out of all included records overall and for five specific EMS subpopulations: cardiac arrest, trauma, stroke, airway intervention, and pediatric patients. RESULTS: We included 36,757,562 EMS records. When applying all criteria, the final algorithm identified 36,552,901 likely episodes of care, representing 99.4% of all records. Most episodes of care (36,353,329, 99.5%) consisted of a single EMS record, with fewer episodes having 2 (195,786, 0.5%), 3 (3,088, <0.1%), 4 (428, <0.1%), and 5 or more (270, <0.1%) records. When evaluating all possible alternative combinations of the criteria, most resulted in a similar number of episodes of care. Among the subpopulations, the number of episodes of care approached the total number of encounters for that condition: cardiac arrest (89.2%-98.5%), pediatric (73.8%-99.3%), stroke (89.5%-99.3%), airway procedure (95.4%-99.4%), and trauma (84.1%-99.6%). Findings were robust to sensitivity analyses which relaxed the time- and age-based criteria. CONCLUSIONS: Nearly all included EMS records in NEMSIS appeared to represent unique patient-care episodes. Approximately 0.5% of episodes involved more than one EMS record, suggesting that related records or multi-unit documentation is uncommon but should be considered in analyses, particularly for selected clinical subgroups.