Introduction: Screening for depression in the emergency department (ED) has been recommended for the last two decades. It is estimated that 1 in 5 adults presenting to the ED meet the criteria for depression, making this setting an ideal point of care for proper and early referral to general practitioners and/or specialist mental health services. One of the barriers to assessment of depression in the ED is a lack of validated tools to screen for depression in this context of use. The purpose of this study is to test the extent to which the commonly used Patient Health Questionnaire (PHQ-9) is valid and reliable to screen for depression in adults presenting to the ED. Methods: Adults, aged 19 years and over, presenting to an inner-city, academic ED with an acute mental health complaint (AMHC) completed a questionnaire package that included demographic questions, the PHQ-9, and 5 other questionnaires for validation purposes. Traditional and Rasch Measurement (RM) methods were applied to the data to examine how well the items: captured the 95% range (±2 logits) of the concept of interest, were reliable and valid, and met the criteria for unidimensional and invariant measurement. Results: Preliminary prospective data from 108/200 adults (mean age 39.7±13.6 years; 65% male) completed the questionnaire package. A total of 58.9% of the sample met the criteria for moderate-severe depression (PHQ-9≥15), with 37% reporting thoughts of suicide and/or self-harm nearly every day for the past two weeks. Analysis of these items showed good overall fit to the Rasch model (χ2=28.3, df=18, p=.06), good reliability (r
=0.84), an ordered 4-point response scale structure, excellent individual item fit, and no item bias for gender, age, level of education, or employment status. Items covered between -1.45 to 1.52 logits, spanning 74% of the targeted theoretical continuum, with gaps at each end of the range. Item #3 (trouble falling or staying asleep) was the easiest item (indicating lower depression) and Items #8 and #9 (moving slowly and thoughts of harm/suicide) were the more difficult items (indicating more severe depression). Conclusion: This study supports the PHQ-9 as a reliable and valid screen for depression in the ED. Incorporating standardized and uniform assessment in Canadian EDs will begin the process of advancing the role of the ED to initiate evidence-based care to optimize the outcomes of Canadians with an AMHC.