Posttraumatic stress disorder and major depressive disorder: Investigatingthe role of overlapping symptoms in diagnostic comorbidity

Studies of posttraumatic stress disorder (PTSD) have found high levels of c omorbid major depressive disorder (MDD). One reason suggested for the comor bidity is the symptom overlap (contaminated symptoms) between the disorders . The present study investigated the contribution of contaminated symptoms (anhedonia, concentration, and sleep problems) to the comorbidity of PTSD a nd MDD. PTSD symptoms were subdivided into two groups: the contaminated sym ptoms and the 14 unique symptoms. It was speculated that if the contaminate d symptoms are responsible for the comorbidity, then they will show less sp ecificity than the unique symptoms, will be less highly correlated with a P TSD symptom total count, and be more frequently endorsed in PTSD patients w ith than without MDD. These hypotheses were tested in a sample (N = 1300) o f psychiatric outpatients, 260 of whom had lifetime PTSD. None of the hypot heses were supported, thereby suggesting that the comorbidity between PTSD and MDD is not an artifact of symptom overlap.