National Surveillance of Emergency Department Visits for Outpatient Adverse Drug Events

Context: Adverse drug events are common and often preventable causes of medical injuries. However, timely, nationally representative information on outpatient adverse drug events is limited.

Objective: To describe the frequency and characteristics of adverse drug events that lead to emergency department visits in the United States.

Design, Setting, And Participants: Active surveillance from January 1, 2004, through December 31, 2005, through the National Electronic Injury Surveillance System-Cooperative Adverse Drug Event Surveillance project.

Main Outcome Measures: National estimates of the numbers, population rates, and severity (measured by hospitalization) of individuals with adverse drug events treated in emergency departments.

Results: Over the 2-year study period, 21,298 adverse drug event cases were reported, producing weighted annual estimates of 701,547 individuals (95% confidence interval [CI], 509,642-893,452) or 2.4 individuals per 1000 population (95% CI, 1.7-3.0) treated in emergency departments. Of these cases, 3487 individuals required hospitalization (annual estimate, 117,318 [16.7%]; 95% CI, 13.1%-20.3%). Adverse drug events accounted for 2.5% (95% CI, 2.0%-3.1%) of estimated emergency department visits for all unintentional injuries and 6.7% (95% CI, 4.7%-8.7%) of those leading to hospitalization and accounted for 0.6% of estimated emergency department visits for all causes. Individuals aged 65 years or older were more likely than younger individuals to sustain adverse drug events (annual estimate, 4.9 vs 2.0 per 1000; rate ratio [RR], 2.4; 95% CI, 1.8-3.0) and more likely to require hospitalization (annual estimate, 1.6 vs 0.23 per 1000; RR, 6.8; 95% CI, 4.3-9.2). Drugs for which regular outpatient monitoring is used to prevent acute toxicity accounted for 41.5% of estimated hospitalizations overall (1381 cases; 95% CI, 30.9%-52.1%) and 54.4% of estimated hospitalizations among individuals aged 65 years or older (829 cases; 95% CI, 45.0%-63.7%).

Conclusions: Adverse drug events among outpatients that lead to emergency department visits are an important cause of morbidity in the United States, particularly among individuals aged 65 years or older. Ongoing, population-based surveillance can help monitor these events and target prevention strategies.