The prevalence and severity of depressive symptoms along the spectrum of unipolar depressive disorders: a post hoc analysis.

Objective: To explore which symptoms are common in patients who experience a range of symptom severity that spans minor depression and major depressive disorder (MDD).

Method: A post hoc analysis of subjects entering outpatient, pharmacologic treatment studies for minor depression or MDD who provided baseline data on the Inventory for Depressive Symptomatology-Clinician Rated (IDS-C) was performed in November 2000. The minor depression sample included 161 patients diagnosed according to the National Institute of Mental Health Diagnostic Interview Schedule, while the MDD subjects included 969 subjects diagnosed according to the Structured Clinical Interview for DSM-III-R. Descriptive statistics were calculated for the total IDS-C score and for each item-rating score for both groups. The percentages of patients within the low, medium, and high severity groups of minor depression and MDD endorsing each IDS-C item were calculated and used to identify specific patterns of prevalence across the 6 groups: symptoms with high prevalence in all groups (core symptoms), those with increasing prevalence across groups (continuum symptoms), and those that become prominent only at a certain threshold of illness severity.

Results: The mean (SD) IDS-C score was 21.18 (5.37) for minor depression patients, while it was 37.14 (7.27) for the MDD patients (P = .0001). Ten items pertaining mostly to mood state and cognition were identified as "core" symptoms of depression based on their high prevalence in all groups. Fourteen items consisting mostly of neurovegetative and somatic symptoms were identified as "continuum" symptoms based on their general pattern of increasing prevalence across the 6 severity groups. Four "threshold" symptoms, including suicidal ideation, psychomotor slowing, gastrointestinal symptoms, and panic/phobic symptoms, were prevalent in only the most severely depressed groups.

Conclusions: The presence of core, continuum, and threshold depressive symptoms indicates central features of both minor depression and MDD as well as symptoms that increase or emerge with depressive illness severity. Some of the core symptoms of minor depression and MDD are not included in DSM depressive criteria or traditional assessment rating scales.