Subthreshold depression as a risk indicator for major depressive disorder: a systematic review of prospective studies

Objective: In order to examine whether the incidence of major depressive disorder (MDD) is increased in subjects with subthreshold depression, or sD (clinically relevant depressive symptoms, without meeting criteria for a full-blown MDD), we conducted a review of prospective studies examining the incidence of MDD in subjects with sD. Method: A systematic literature search was conducted. For all studies, the relative risk of developing MDD was calculated, based on person-years. Results: Twenty studies (23 comparisons) were found, based on community samples, general medical patients and high-risk subjects. Most comparisons showed that subjects with sD had a consistently larger chance of developing MDD. The studies differed considerably in the definition of sD, the recency (occurrence of the last sD) and the in-/exclusion of lifetime MDD. Conclusion: The incidence of MDD in subjects with sD is larger than in subjects without sD. Otherwise, the concept of sD is too broad to be used. In future studies, some consensus should be reached regarding the definition of sD.