Identifying geographical regions serviced by hospitals to assess laboratory-based outcomes

Objective: To define geographical regions (forward sortation areas; FSAs) in Southwestern Ontario, Canada from which patients would reliably present to a hospital with linked laboratory data if they developed adverse events related to medications dispensed in outpatient pharmacies. Design: Descriptive research. Setting: Forty-five hospitals in Southwestern Ontario, Canada, from 2003 to 2009. Participants: Patients aged 66 years and older who received an outpatient prescription for any drug and presented to the emergency department in the subsequent 120 days. Main outcome measure: The proportion of patients in a given FSA presenting to an emergency department at a hospital with linked laboratory data versus a hospital without linked laboratory data. To be included in the catchment area at least 90% of emergency department visits in an FSA must have occurred at laboratory-linked hospitals in a given year. Results: Over the study period, there were 649 713 emergency department visits by patients with recent prescription claims from pharmacies in 1 of 118 FSAs. In total, 141 302 of these patients presented to an emergency department at a laboratory-linked hospital. For the year 2003, 12 FSAs met our criteria to be in the catchment area and this number grew to 25 FSAs by the year 2009. Conclusions: The relevant geographical regions for hospitals with linked laboratory data have been successfully identified. Studies can now be conducted using these well-defined areas to obtain reliable information on the incidence and absolute risk of presenting to hospital with laboratory abnormalities in older adults dispensed commonly prescribed medications in outpatient pharmacies.