We know of no clinical trial in which patients screened for depression had better outcomes than patients who were not screened when the same resources for treatment were available to both groups, as would be the case in primary care settings. Should we nonetheless assume that routine screening would benefit patients? Several factors argue against that assumption.
Screening can generally only succeed to the degree that it identifies patients whose condition is otherwise unrecognized and untreated. Yet, prescription rates for antidepressant medications are already high and are trending upward.26
Among adults 35 years of age and older in the US, annual rates of use for antidepressant medications increased from 8.3% to 14.1% from 1996 to 2005, with one-third to one-half of prescriptions specifically for psychiatric problems.26
In a 2005 study from Canada, 7% of a sample of the general population reported current use of antidepressant medications, a higher figure than the estimated prevalence of major depression (4%).27
Rates of prescriptions for antidepressant medications may be even higher among patients with long-term diseases. In Ontario, the rate of prescriptions for antidepressant medications among patients aged 65 years and older within six months of having an acute myocardial infarction doubled from 8% in 1993 to 16% in 2002.28
Beyond treatment with antidepressant medications, province-wide data from Quebec show that 23% of women and 19% of men who consulted a physician in 2003 received a diagnostic billing code covering a depression or anxiety disorder at some point during the year.29
Indeed, as the frequency of recognition and treatment increases, the yield of screening interventions will decrease.
A consequence of the increasing rate of treatment for depression is that existing studies appear to exaggerate the accuracy of screening questionnaires and the number of otherwise unrecognized patients who would be identified through screening. A recent overview evaluated original studies on the accuracy of depression screening tools that were included in 17 systematic reviews or meta-analyses published from 2005 to 2009.30
The authors found that 189 of 197 original studies in those reviews (95.9%) included patients with depression that had already been diagnosed or was being treated. Screening, however, is intended to identify new cases. Including patients whose depression has already been diagnosed or treated in studies of screening tools exaggerates the ability of the tools to identify new cases and the number of people who would start treatment as a result of the screening process. Indeed, the authors of the overview estimated that the number of patients with untreated depression who would actually be detected by screening may be less than half the number predicted by existing studies.
In addition, the authors concluded that more than 80% of positive screening results are likely false-positives in primary care settings. This estimate was based on published estimates of sensitivity (85%) and specificity (74%) in primary care31
and a 10% prevalence of major depression,32
conservatively assuming that 50% of cases in actual practice are recognized without screening33,34
and that published sensitivity figures are overestimated by 10% owing to the inclusion of patients whose condition has already been diagnosed or treated.30
Thus, when used to identify patients whose condition is undetected and untreated, screening tools may be much less accurate than is usually assumed. Compounding this problem, no studies have estimated the cumulative false-positive rate from periodic screening for depression in primary care, which would certainly be very high.
Recent evidence raises questions about the degree to which standard treatments for depression may benefit patients who are identified through screening, but whose condition is not obvious enough for them to seek help or be otherwise identified. Recent meta-analyses have reported that the effects of antidepressant medications are smaller than previously believed when all studies, rather than just published studies, are included (74 studies, SMD 0.31, 95% CI 0.27–0.35).35
In addition, when compared with patients who receive a placebo, patients with minor depression or mild symptoms of major depression receive much less benefit, if any, from treatment with antidepressant medications than patients with more severe symptoms.36–40
There is less evidence specific to primary care settings; however, results generally appear to be similar,25
despite the potentially increased heterogeneity and lower severity of depression in these settings. Consequently, the UK guidelines7
recommend against using antidepressant medications as a first-line therapy for mild depression in primary care, citing a poor risk–benefit ratio. Psychotherapy is used much less frequently than drug therapy in primary care settings, but estimated effectiveness was generally similar to drug therapies when considering only studies that met minimum quality criteria in eight areas (11 trials, SMD 0.22, 95% CI 0.13–0.31)41
or accounting for likely publication bias (117 trials, SMD 0.42, 95% CI 0.33–0.51).42
Authors of a recent meta-analysis43
found that, in primary care, psychological treatments were effective when patients were referred by their primary care physician (SMD = 0.43, 95% CI 0.28 to 0.58), but not when patients were recruited through screening (SMD = 0.13, 95% CI −0.08 to 0.34). As with antidepressant medications, there is evidence that psychological treatments may be less effective for patients with only mild symptoms of depression.44