Depression can be conceptualized along a continuum of severity from subthreshold or minor depression to MDD of varying severity (e.g., mild, moderate, and severe) (17). Using a continuum approach may augment statistical power as sample size can be increased substantially and patients who fall into the ‘grey area’ can be assessed. Several lines of evidence support a depression continuum. In longitudinal studies, there is an increased risk of MDD in patients with minor depression and subthreshold depression (18, 19). Statistical studies of disorder classification (taxometric) suggested that severity of depression is continuously distributed and there is no discontinuity in the latent structure of depression (19, 20). Family studies report that relatives of probands with milder forms of depression have greater risk of MDD compared to relatives of probands without any mood disorders (21–24). A higher number of depressive symptoms is related to greater disability, worse quality of life, and a higher mortality risk (18, 25–29). MDD and continuous measures of depression are highly correlated and severity of depressive symptoms along the continuum is linear (30, 31).
