Subthreshold depression based on functional impairment better defined by symptom severity than by number of DSM-IV symptoms

Background: Depression with fewer symptoms than required for a DSM-IV diagnosis of Major Depressive Disorder (MDD) has consistently been found to be associated with functional impairment. In this study, we aim to define clinically significant depression below the DSM-IV threshold for Major Depressive Disorder (MDD) by means of functional impairment.

Methods: Data used are from 2157 respondents of the Netherlands Study of Depression and Anxiety (NESDA). The Composite International Diagnostic Interview (CIDI) and the Inventory for Depressive Symptomatology-Self Report (IDS-SR(30)) were compared in their association with functional impairment as measured by the World Health Organization Disability Assessment Schedule II (WHODAS II). We used ANCOVA, adjusting for gender, age, education and somatic conditions, and ROC analyses.

Results: The IDS-SR(30) (p<.001, eta(2)=.51) was more strongly associated with functional impairment than CIDI symptom count (p<.001, eta(2)=.035). Effect sizes supported four symptoms on the CIDI, and a score within the mild depression range on the IDS-SR(30) as adequate cut-offs for defining subthreshold depression, respectively. ROC analyses showed that these cut-offs identified the top 10% and 8% to 60% most dysfunctional respondents, respectively.

Limitations: Suggested cut-offs seem reasonable on the basis of converging findings, but in lack of a golden standard they remain somewhat arbitrary. Furthermore, the design of the study is cross-sectional in nature, no causal inferences between depression and functional impairment can be made.

Conclusions: Although both instruments are associated with functional impairment, the IDS-SR(30) seems better suited than the CIDI to define subthreshold depression, with a cut-off in the mild IDS-SR(30) range.