Objectives: Although major depression is characteristic of both bipolar disorder and major depressive disorder, there is disagreement as to whether there are distinct features of depression that differentiate these two conditions. The primary aim of this study was to use methods based in item response theory to evaluate differences in DSM-IV depression symptom endorsement in an epidemiological sample of individuals with a history of mania (i.e., bipolar depression) versus those without (i.e., unipolar depression).
Methods: Clinical interview data were drawn from a subsample (n = 13,058) of individuals with bipolar or unipolar depression who had participated in the National Epidemiologic Survey on Alcohol and Related Conditions. Using these data, a two-parameter item response model was used to estimate differential item functioning of DSM-IV depressive symptoms between these two groups.
Results: Differences in severity parameter estimates revealed that suicidal ideation and psychomotor disturbance were more likely to be endorsed across most levels of depression severity in bipolar versus unipolar depression. Differences in discrimination parameter estimates revealed that fatigue was significantly less discriminating in bipolar versus unipolar depression.
Conclusions: Equating for level of depression symptom severity, study results revealed that suicidal ideation and psychomotor disturbance are endorsed more frequently in bipolar versus unipolar depression. Study data also suggested that fatigue may be endorsed more frequently in unipolar relative to bipolar samples at moderate (versus low or high) levels of depression symptom severity.