Differential item functioning of DSM-IV depressive symptoms in individuals with a history of mania versus those without: an item response theory analysis

Depression is a key feature of both major depressive disorder (MDD) and bipolar disorder. Although some individuals with bipolar disorder do not experience depression, accumulating evidence suggests that over the long term, the majority of those with bipolar disorder will exhibit depression or depressive symptoms much more frequently than manic symptoms (1, 2). Initial misdiagnosis of MDD is quite common among patients with bipolar disorder (3), which may delay appropriate treatment and, in the case of antidepressant monotherapy, potentially exacerbate mood cycling in vulnerable populations (4). Given this risk for misdiagnosis, it is important to identify whether there are differences in the symptomatic presentation of depression in MDD versus bipolar disorder. If differences exist, understanding them will assist in differential diagnosis and increase the likelihood that a patient receives pharmacological treatment appropriate to his or her illness. Further, an understanding of differences in the phenomenology between bipolar and unipolar depression will be important for creating and adapting psychosocial treatments for bipolar depression. Although there have been several studies that have attempted to identify differences between depression in MDD versus bipolar disorder, a review of the literature reveals somewhat contradictory and inconclusive findings. Indeed, some (5–9), but not all (10), have reported a greater prevalence of atypical features in bipolar versus unipolar depression. Yet others have reported a greater prevalence of melancholic symptoms in bipolar depression (8, 11). Additional research suggests that unipolar depression may be more strongly characterized by anxiety and somatization (12) and that bipolar depression may be more strongly associated with psychosis (8, 11). Although such related clinical features may, indeed, be informative for differentiating the two syndromes, they are nevertheless not DSM-IV-defined core symptoms of major depression. Thus, such features are limited in informing our understanding of how the core symptoms of depression differentially operate in the two groups. For example, it is possible that a greater prevalence of psychosis in bipolar depression may be better accounted for by a history of mania. Finally, a recent review covering over 30 years of research concluded that unipolar and bipolar depression symptom presentations are more similar than different (12), and others have recently argued that the two cannot be differentiated (13). This mixed evidence may be related to methodological limitations in the existing literature. Primarily, the large majority of these studies have utilized clinical versus community samples of individuals (12), some of whom had been enrolled in clinical efficacy trials that use narrow inclusion criteria (14, 15). In addition, with few exceptions (11, 15), sample sizes have been generally small, thus rendering estimates unstable. Further, inconsistencies across studies may be accounted for by Type I error, as most studies have performed a large number of statistical comparisons without corresponding alpha level correction (12). Finally, several studies have not controlled for overall symptom severity in their comparisons across groups (5, 6, 11, 12, 14, 16). Thus, it is unclear whether any differential symptom expression is due to true phenomenological differences between bipolar and unipolar depression, or whether such differences are reflective of greater overall depression symptom severity in one group versus another. To address these limitations, the current study used methods based in item response theory (IRT) (17) to evaluate differences in DSM-IV depression symptom endorsement in a community sample of individuals with bipolar versus unipolar depression. IRT methodology provides significant improvement over previously used techniques, as it allows one to examine the likelihood that a particular symptom will be endorsed at a particular level of depression severity. Thus, differences between bipolar and unipolar depression can be evaluated while simultaneously equating for depression symptom severity. Further extending the literature, we conducted analyses using a large, nationally representative sample of individuals.