Subthreshold depression in adolescence and mental health outcomes in adulthood.

BACKGROUND There is increasing interest in the extent to which individuals with subthreshold depression face increased risks of subsequent major depression and other disorders. OBJECTIVE To examine linkages between the extent of depressive symptoms (asymptomatic, subthreshold, major depression) at ages 17 to 18 years and mental health outcomes up to age 25 years in a New Zealand birth cohort. DESIGN Data were gathered during the Christchurch Health and Development Study, a 25-year longitudinal study of a birth cohort of 1265 New Zealand children (635 males, 630 females). SETTING General community sample. PARTICIPANTS The analysis was based on 1006 participants who represented 80% of the original cohort. MAIN OUTCOME MEASURES Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition symptom criteria for major depression and anxiety disorder, treatment-seeking, suicidal ideation, and suicide attempt. RESULTS There were significant associations (P<.01) between the extent of depression at ages 17 to 18 years and rates of subsequent depressive symptoms, major depression, treatment for depression, anxiety disorder, treatment for anxiety disorder, suicidal ideation, and suicide attempts. After adjustment for covariate factors, the extent of depression at ages 17 to 18 years remained associated with later depression and suicidal tendencies. Planned comparisons showed that sample members with subthreshold depression had a similar prognosis to those meeting criteria for major depression. CONCLUSIONS Findings suggest that sample members with subthreshold depression are a group with elevated risks of later depression and suicidal behaviors. Current diagnostic procedures, which classify people with subthreshold depression into complex discrete groups, might obscure the fact that depressive symptoms are dimensional and range from none to severe.