To determine the association between functional impairment, as reported in a lay-administered structured interview (CIDI), and severity of depression, depressive symptoms and risk factors for depression.

We undertook a cross-sectional study of 5442 consecutive attendees at general practices in seven Spanish provinces participating in the PredictD-Spain study on predictors of depression. Participants were administered the depression section of the Composite International Diagnostic Interview (WHO-CIDI 2.1), allowing diagnoses by the ICD-10 and DSM-IV classifications for depressive episodes. Impairment was measured using the CIDI question about whether symptoms seriously interfered with important areas of functioning, such as work or looking after the house and family. We measured a set of 39 known risk factors for depression.

Firstly, the 6-month prevalence of a depressive episode according to ICD-10 was 28.7% (1563) and of major depression according to DSM-IV it was 13.6% (742). Secondly, out of the 1563 patients with a depressive episode according to ICD-10, nearly half (47.9%; n=749) had no impairment in important areas of functioning.

As the ICD-10 criteria for depressive diagnoses do not include the criteria that symptoms cause impairment in social, occupational or other important areas of functioning, a large number of false positive cases are included in reported prevalence rates; and secondly, the measurement of functional impairment, at least operationalized using a lay-administered structured interview such as CIDI, is not enough, in epidemiological research studies, to assess the clinical importance of depressive symptoms.