Keywords drug-related side effects and adverse reactions, emergency service, health care, hospital, quality indicators, trigger tools AIMSTrigger tools are retrospective surveillance methods that can be used to identify adverse drug events (ADEs), unintended and harmful effects of medications, in medical records. Trigger tools are used in quality improvement, public health surveillance and research activities. The objective of the study was to evaluate the performance of trigger tools in identifying ADEs. METHODSThis study was a sub-study of a prospective cohort study which enrolled adults presenting to one tertiary care emergency department. Clinical pharmacists evaluated patients for ADEs at the point-of-care. Twelve months after the prospective study's completion, the patients' medical records were reviewed using eight different trigger tools. ADEs identified using each trigger tool were compared with events identified at the point-of-care. The primary outcome was the sensitivity of each trigger tool for ADEs. RESULTSAmong 1151 patients, 152 (13.2%, 95% confidence intervals (CI) 11.4, 15.3%) were diagnosed with one or more ADEs at the point-of-care. The sensitivity of the trigger tools for detecting ADEs ranged from 2.6% (95% CI 0.7, 6.6%) to 15.8% (95% CI 10.6, 22.8%). Their specificity varied from 99.3% (95% CI 98.6, 99.7) to 100% (95% CI 99.6, 100%). CONCLUSIONThe trigger tools examined had poor sensitivity for identifying ADEs in emergency department patients, when applied manually and in retrospect. Reliance on these methods to detect ADEs for quality improvement, surveillance, and research activities is likely to underestimate their occurrence, and may lead to biased estimates.