Posttraumatic stress disorder and major depressive disorder: investigating the role of overlapping symptoms in diagnostic comorbidity.

Studies of posttraumatic stress disorder (PTSD) have found high levels of comorbid major depressive disorder (MDD). One reason suggested for the comorbidity 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 and MDD. PTSD symptoms were subdivided into two groups: the contaminated symptoms and the 14 unique symptoms. It was speculated that if the contaminated symptoms are responsible for the comorbidity, then they will show less specificity than the unique symptoms, will be less highly correlated with a PTSD symptom total count, and be more frequently endorsed in PTSD patients with than without MDD. These hypotheses were tested in a sample (N = 1300) of psychiatric outpatients, 260 of whom had lifetime PTSD. None of the hypotheses were supported, thereby suggesting that the comorbidity between PTSD and MDD is not an artifact of symptom overlap.