Methodology This prospective observational study will evaluate predictor variables (from the history, physical examination and laboratory tests) for ADREs in a representative sample of ED patients using standardized assessments by emergency nurses and physicians. Inter-rater agreement of predictor variables will be measured, and clinical pharmacists, blinded to the nursing and physician assessments, will use a standardized, validated algorithm to identify ADREs. Recursive partitioning and/or logistic regression analysis will be used to determine the optimal combination of predictor variables to detect the presence of an ADRE. We will report diagnostic test characteristics of the derived clinical decision rule. Validation of the clinical decision rule is planned in a future study.