Dimensionality and the category of major depressive episode

Major depressive episode (MDE) is a chronic disease typified by episodes that remit and recur. It is a major contributor to the burden of disease. The diagnosis of a disorder is an expert opinion that the disorder is present. The nine symptoms of MDE exist on dimensions of greater or lesser intensity, persistence over time, change in usual state, distress and impairment. It is the clinician's task to judge whether the elicited symptoms warrant the diagnosis. The surprise is that trained clinicians can do this reliably and that diagnostic interviews and questionnaires can emulate this process. The distribution of symptoms in community surveys is exponential, with no obvious discontinuity at the diagnostic threshold. Taxometric and primary care studies confirm this. The number of symptoms predicts severity, comorbidity, family history, disability, help seeking and treatment recommendations. The latent structure of mental disorders places MDE in the distress misery cluster. Measures of well-being, distress, disability and neuroticism correlate with the number of symptoms but the relation is not perfect. The Patient Health Questionnaire is derived from the diagnostic criteria and does not suffer this limitation. The introduction of measures like this would acknowledge dimensionality, would facilitate recognition, guide treatment, and be acceptable to consumers, providers and funders.