A tetrachoric factor analysis validation of mixed depression.

BACKGROUND: Mixed depression, i.e. a Major Depressive Episode plus co-occurring manic/hypomanic symptoms, has recently become the focus of research. However, its diagnostic validity and bipolar nature are still not firmly supported. A bipolar nature could have significant treatment impacts.

STUDY AIM: The aim was to psychometrically validate the concept of, and the bipolar nature, of mixed depression, by using (for the first time) tetrachoric factor analysis of its hypomanic symptoms.

METHODS: Consecutive 441 Bipolar II Disorder (BP-II), and 289 Major Depressive Disorder (MDD) outpatients were cross-sectionally assessed for Major Depressive Episode (MDE) and concurrent hypomanic symptoms (as binary variables) when presenting for treatment of depression, by a mood disorder specialist psychiatrist (FB), using the Structured Clinical Interview for DSM-IV (as modified by [Akiskal HS, Benazzi F. Optimizing the detection of bipolar II disorder in outpatient private practice: toward a systematization of clinical diagnostic wisdom. J Clin Psychiatry 2005; 66: 914-921.]) in a private practice. Consecutive 275 remitted BP-II were also assessed for past hypomania. Mixed depression was defined as co-occurrence of MDE and 3 or more, usually subthreshold, hypomanic symptoms.

RESULTS: In multivariable logistic regression, BP-II independent predictor variables were young onset age, MDE recurrences, mixed depression, and bipolar family history. Factor analysis of past hypomania symptoms found three factors: an "irritable mental overactivity" factor, an "elevated mood" factor, and a "motor overactivity" factor. Factor analysis of intradepression hypomanic symptoms in BP-II, and in MDD, found two similar mental and motor overactivity factors. Multivariate regression of the intradepression hypomanic factors versus bipolar validators, such as bipolar family history and young onset age, found significant associations.

DISCUSSION: Findings could support the diagnostic validity, and the bipolar nature, of mixed depression, on the basis of the close similarities found between the factor structure of inter-depression hypomania and intra-depression hypomanic symptoms. Impacts on treatment of a bipolar nature of mixed depression may be significant (e.g. more use of mood stabilising agents, less/no use of antidepressants).