Few studies have been conducted looking at clinical features associated to treatment resistant depression (TRD) defined as failure to at least two consecutive antidepressant trials. The objective of this study was to identify clinical and demographic factors associated to TRD in a large sample of depressed patients who failed to reach response or remission after at least two consecutive adequate treatments.

A total of 702 patients with unipolar major depression were included in the analysis. 346 patients were considered as non resistant. The remaining 356 patients were considered as resistant with a HAM-D-17 score remaining ≥ 17 after 2 consecutive adequate trials. Cox regression models were used to examine the association between individual clinical variables and TRD.

Eleven variables were found to be associated with TRD. Anxiety comorbidity (p<0.001, OR=2.6), comorbid panic disorder (p<0.001, OR=2.6) and social phobia (p<0.008, OR=2.1), personality disorder (p<0.049, OR=1.7), suicidal risk (p<0.001, OR=2.2), severity (p<0.001, OR=1.7), melancholia (p<0.018, OR=1.5), a number of hospitalizations > 1 (p<0.003, OR=1.6), recurrent episodes (p<0.009, OR=1.5), early age of onset (p<0.009, OR=2.0) and non response to the first antidepressant received lifetime (p<0.019, OR=1.6).

Our findings provide a set of eleven relevant clinical variables associated to TRD which can be explored at the clinical level. The statistical model used in this analysis allowed for a hierarchy of these variables (based on the OR) showing that comorbid anxiety disorder is the most powerful clinical factor associated to TRD.