Defining a threshold above which an adult can be considered to frequently use ambulance services: a retrospective cross-sectional study of emergency calls to an ambulance service in England.

Objective: There is no empirical definition of adult frequent use of ambulance services. This study aimed to define a threshold, and utilise this to explore characteristics of people frequently using services. Methods: This was a retrospective cross-sectional study in a single ambulance service in England. Routinely collected, pseudo-anonymised call- and patient-level data were collected for two months (January and June 2019). Incidents, defined as independent episodes of care, were analysed using a zero-truncated Poisson regression model to determine a suitable frequent-use threshold, with comparisons subsequently made between frequent and non-frequent users. Results: A total of 101,356 incidents involving 83,994 patients were included in the analysis. Two potentially appropriate thresholds were identified: five incidents per month (A); and six incidents per month (B). Threshold A produced 3137 incidents from 205 patients, with five patients likely false-positive identifications. Threshold B produced 2217 incidents from 95 patients, with no false-positive identifications but 100 false-negatives compared to threshold A. Regardless of threshold, frequent users compared to non-frequent users had relatively reduced service use between 08:00 and 15:00, were younger and were more likely to receive lower-priority responses (all p < 0.001). We identified several chief complaints indicative of increased frequent use, including chest pain, psychiatric/suicide attempt and abdominal pains/problems. Conclusions: We suggest a threshold of five incidents per month, with recognition that a small number of patients may be incorrectly identified as using ambulance services frequently. The rationale for this choice is discussed. This threshold may be applicable in wider UK settings and could be used for the routine automated identification of people using ambulance services frequently. The identified characteristics can help inform interventions. Future research should examine applicability of this threshold in other UK ambulance services and countries where patterns and determinants of frequent ambulance use may differ.