Distinguishing subclinical (subthreshold) depression from the residual symptoms of major depression.

Abstract

The terms ‘sub-clinical’ or ‘subthreshold’ are widely used in medicine to label individuals who are in the early stages of a disease process (e.g., cancer, hypertension, etc.) and to identify high-risk populations that need to be monitored or provided with specific preventative interventions or treatments. Because the pathophysiological changes that occur in the sub-clinical stages of a condition are similar to those that occur during the full-blown disease, sub-clinical states are usually considered prodromal versions of the disease under consideration. Based on this usage, subclinical or subthreshold depression should refer to an individual who has not previously met full criteria for major depression who currently experiences depressive symptoms that are not severe enough or persistent enough to merit a diagnosis of major depression. Using this concept, a recent two-year follow-up study found that subthreshold depression can predict the subsequent occurrence of depressive disorders (OR=6.23, 95% CI=3.31-11.74) or anxiety disorders (OR=3.47, 95% CI= 1.78-6.78).[1] Unlike other medical disciplines, in clinical psychiatry the sub-clinical and subthreshold labels are also used to describe residual clinical states after the partial remission of a full episode of a condition. For example, ‘sub-clinical depression’ is often used to describe the clinical state of a patient who has not entered full remission and still has residual symptoms after treatment with antidepressants.[2],[3] I believe it important to distinguish these two situations. Prodromal depressive symptoms prior to the onset of a full-criteria major depression and the residual depressive symptoms after the partial resolution of a major depressive episode may be symptomatically similar but they are clinically distinct conditions that require different interventions so it is inappropriate to combine them under a single label. Residuals symptoms following a major depressive illness require continued pharmacological and/or psychotherapeutic treatment but sub-clinical depressive symptoms prior to the first occurrence of a major depressive illness usually only require observation and follow-up. More active treatment for sub-clinical depressive symptoms is usually only justified if they persist for two years or longer (in which case the diagnosis of Dysthymic Disorder is usually applied) or if the individual has had full-criteria major depressive illness in the past and thus is at a high risk of progressing to a full-criteria episode of illness. Collapsing these different situations under a single rubric is both inaccurate and could potentially lead to inappropriate treatment. I recommend that persistent depressive symptoms following a major depressive illness that do not meet the full criteria of the disorder be labeled ‘residual symptoms’ (not sub-clinical or subthreshold symptoms) and that the ‘sub-clinical depression’ or ‘subthreshold depression’ labels be limited to depressive symptoms that occur prior to the first onset of a full depressive episode. But depressive affect is part of the normal human condition so we must also avoid the inappropriate overuse of the sub-clinical and subthreshold labels by, for example, labeling the depressive symptoms identified in screening programs of healthy community members as ‘subthreshold depression’. Depressive symptoms in individuals without prior episodes of full-criteria depressive illness should only be given the sub-clinical or subthreshold labels if the symptoms are ‘clinically significant’, that is, when they result in significant distress or significant social or occupational dysfunction.