The DSM-IV definition of severity of major depression: inter-relationship and validity

Background. Severity is an important characteristic of major depression (MD) and an 'episode specifier' in DSM-IV classifying depressive episodes as 'mild', 'moderate' or 'severe'. These severity subtypes rely on three different measures of severity : number of criteria symptoms, severity of the symptoms and degree of functional disability. No prior empirical study has evaluated the coherence and validity of the DSM-IV definition of severity of MD. Method. In a sample of 1015 (518 males, 497 females) Caucasian twins from a population-based registry who met criteria for MD in the year prior to interview, factor analysis and logistic regression were conducted to examine the inter-relationships of the three severity measures and their associations with a wide range of potential validators including demographic factors, risk for future episodes, risk of MD in the co-twin, characteristics of the depressive episode, the pattern of co-morbidity, and personality traits. Results. Correlations between the three severity measures were significant but moderate. Factor analysis indicated the existence of a general severity factor, but the factor was not highly coherent. The three severity measures showed differential predictive ability for most of the validators. Conclusions. Severity of MD as defined by the DSM-IV is a multifaceted and heterogeneous construct. The three proposed severity measures reflect partly overlapping but partly independent domains with differential validity as assessed by a wide range of clinical characteristics. Clinicians should probably use a combination of severity measures as proposed in DSM-IV rather than privileging one.