[Estimation of prevalence and treatment needs of mental disorders. The problem of diagnostic thresholds]

Uncertainties in the context of threshold-based diagnostics represent a theoretically unsolved methodological problem that may require multidimensional solutions. Pragmatically, current research focuses on establishing reliable and valid operationalized criteria within the framework of diagnostic systems, such as the International Classification of Diseases (ICD-10) and the Diagnostic and Statistical Manual of Mental Disorders (DSM).

By means of model calculations based on epidemiological data we show how exemplified changes in the disorder spectrum and diagnostic criteria influence case numbers. Furthermore, we investigate how threshold-based constructs, such as DSM-IV diagnoses, relate to the general criteria of illness and sickness.

Variations in the disorder spectrum and thresholds lead to slight to moderate changes in case numbers. Regarding distress and impairment, mental disorders are associated with significantly reduced health-related quality of life and an increased number of days out of role (due to mental and/or physical problems). With increasing distress and impairment, the percentage of mental disorders increases significantly; in the 5 % of the general population with the highest distress and impairment, the proportion is nearly 80 %.

Despite fuzzy boundaries, threshold-based diagnoses (DSM-IV) represent a satisfactory and reproducible disease classification (in terms of illness and sickness) for estimation of prevalence. There is a lack of definitions and instruments to assess treatment needs. It is still debated whether diagnostic symptom criteria always represent pathological disorders (i. e. disease).