Is clinical depression distinct from subthreshold depressive symptoms? A review of the continuity issue in depression research

Resolving whether subthreshold depressive symptoms exist on a continuum wit h unipolar clinical depression is important for progress on both theoretica l and applied issues. To date, most studies have found that individuals wit h subthreshold depressive symptoms resemble cases of major depressive disor der along many important dimensions (e.g., in terms of patterns of function al impairment, psychiatric and physical comorbidity, familiality, sleeping EEG, and risk of future major depression). However, such manifest similarit ies do not rule out the possibility of a latent qualitative difference betw een subthreshold and diagnosable depression. Formal taxonomic analyses, int ended to resolve the possibility of a latent qualitative distinction, have so far yielded contradictory findings. Several large-sample latent class an alyses (LCA) have identified latent clinical and nonclinical classes of uni polar depression, but LCA is vulnerable to identification of spurious class es. Paul Meehl's taxometric methods provide a potentially conservative alte rnative way to identify latent classes. The one comprehensive taxometric an alysis reported to date suggests that self-report depression symptoms occur along a latent continuum but exclusive reliance on self-report depression measures and incomplete information regarding sample base rates of depressi on makes it difficult to draw strong inferences from that report. We conclu de that although most of the evidence at this time appears to favor both a manifest and latent continuum of unipolar depression symptomatology, severa l important issues remain unresolved. Complete resolution of the continuity question would be speeded by the application of both taxometric techniques and LCA to a single large sample with a known base rate of lifetime diagno sed depressives.