Definition and epidemiology of treatment-resistant depression

Although many depressed patients do improve with adequate antidepressant treatment, a significant proportion fails to reach acceptable levels of functioning and well-being. Currently, however, there is no universally accepted definition of treatment resistance in unipolar depression (TRD). This article proposes an operational definition of TRD and discusses partial response, nonresponse, and methodologic issues that emerge in the assessment of resistance. Finally, possible predictors of failure to respond are reviewed, and an attempt is made to estimate the prevalence of TRD.