The prevalence, clinical relevance, and public health significance of subthreshold depressions.

Scientific evidence has accumulated during the last 15 years establishing that SD symptoms have a high prevalence in the general population and in clinically depressed patient cohorts studied cross-sectionally or followed longitudinally. The clinical relevance and public health importance of SD symptoms were confirmed when various investigators, including the authors' group at University of California, San Diego, found that SD symptoms are associated with a significant and pervasive impairment of psychosocial function when compared to no depressive symptoms. There is strong evidence that all levels of depressive symptom severity of unipolar MDD are associated with significant psychosocial impairment, which increases significantly and linearly with each increment in level of symptom severity. It is only when MDD patients are completely symptom free that psychosocial function returns to good or very good levels. The disability associated with depression is state dependent, and disability returns to good or normal levels only when all of the depressed patients' symptoms abate, because disability is present when even a few symptoms (i.e., SD symptoms) are detected. There is strong evidence during the long-term course of illness that major, minor, dysthymic, and subsyndromal symptoms wax and wane within the same patient and that these symptomatic periods are interspersed in the overall course with times when patients are remitted and symptom free. The modal longitudinal symptom status of MDD patients involves primarily subthreshold depressive symptoms, which are much more common than symptoms at the syndromal MDE level. The longitudinal systematic examination of the clinical relevance and high prevalence of SD symptoms helped establish the fact that the long-term symptomatic expression of MDD is dimensional, not categorical, in nature. Abatement of SD symptoms is of fundamental importance in defining full remission or recovery of MDEs. Ongoing residual SD symptoms during the recovery periods after an MDE are associated with psychosocial disability, more rapid MDE relapse, and a more severe chronic future course of illness, all of which indicate that when residual SD symptoms are present the MDE has not fully remitted and the disease is still active. When all depressive symptoms of an MDE abate for a minimum of 8 weeks, then full remission has been achieved. MDE remission defined in this way is associated with significant delay or even prevention of future episode relapse and a less severe, relapsing, and chronic future course. The authors submit that the research reviewed in this article heralds a new paradigm in understanding the progression of clinical depression through various overlapping stages of severity, which begin at the seemingly "subclinical" level of depressive symptoms. This conceptualization in turn dictates a public health approach, which emphasizes that treatment of MDD even at the deceptively mild levels of symptoms should be initiated or maintained.