Background: The ICD-10 categorisation of severity of depression into mild, moderate and severe depressive episodes has not been validated.
Aims: To validate the ICD-10 categorisation of severity of depression by estimating its predictive ability on the course of illness and suicidal outcome.
Method: All psychiatric in-patients in Denmark who had received a diagnosis of a single depressive episode at their first discharge between 1994 and 1999 were identified. The risk of relapse and the risk of suicide were compared for patients discharged with an ICD-10 diagnosis of a single mild, moderate or severe depressive episode.
Results: At their first discharge, 1103 patients had an ICD-10 diagnosis of mild depressive episode, 3182 had a diagnosis of moderate depressive episode and 2914 had a diagnosis of severe depressive episode. The risk of relapse and the risk of suicide were significantly different for the three types of depression--increasing from mild to moderate to severe depressive episode.
Conclusions: The ICD-10 way of grading severity is clinically useful and should be preserved in future versions.