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 with unipolar clinical depression is important for progress on both theoretical and applied issues. To date, most studies have found that individuals with subthreshold depressive symptoms resemble cases of major depressive disorder along many important dimensions (e.g., in terms of patterns of functional impairment, psychiatric and physical comorbidity, familiality, sleeping EEG, and risk of future major depression). However, such manifest similarities do not rule out the possibility of a latent qualitative difference between subthreshold and diagnosable depression. Formal taxonomic analyses, intended to resolve the possibility of a latent qualitative distinction, have so far yielded contradictory findings. Several large-sample latent class analyses (LCA) have identified latent clinical and nonclinical classes of unipolar depression, but LCA is vulnerable to identification of spurious classes. Paul Meehl's taxometric methods provide a potentially conservative alternative way to identify latent classes. The one comprehensive taxometric analysis 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 depression makes it difficult to draw strong inferences from that report. We conclude that although most of the evidence at this time appears to favor both a manifest and latent continuum of unipolar depression symptomatology, several 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 diagnosed depressives.