Limitations of the Studies
In the community studies subjects were identified with screening instruments. Therefore, a number of false-positive diagnoses may have been included in these studies that biased outcome results.47 This risk was minimized by using interviews in addition to the screening instruments. Another important limitation is the risk of missing part of the information about recurrences in the intervals between assessments. This risk is less in studies that include more than one follow-up assessment, as was done in 2 of the included studies,41,42 and also in studies where data about the interval are retrieved using information based on patients’ recall.40,43 Recall is known to introduce bias by not always giving sufficient details after longer periods of time.48
In the family practice studies where the specificity of the diagnosis is usually high,49,50 outcome results may have been biased because the results of undetected or misdiagnosed patients with depression are missing. In both studies the information was retrieved from the case records. Thus, accuracy depended on the completeness of the physicians’ notes. In one of the studies the case records were used in addition to data from a morbidity registry in which physicians were trained regularly to use criteria for diagnosis. Although suicide data can be found in this study, patients who left the practices or died within 10 years were excluded, and outcome results should be viewed taking that into consideration.
An important shortcoming is that most studies started at first or recurrent episodes, so it is not possible to give an exact percentage of single-episode depression versus recurrent illness. A description of the longitudinal course starting at first diagnosis is also not possible. Even in the one study starting at the first episode we can only draw conclusions about recurrence rates after diagnosis, because we have no certainty that the first diagnosis was in fact related to the first depressive episode. Other shortcomings are the small number of patients in the follow-up in 2 studies39,43 and uncertainty about the representativeness of the samples in one of the community studies.39,41,42,44 Since a family history of depression is a risk factor in an individual,51 the population of family members of affectively ill probands cannot be regarded as a representative community sample.44
The validity of the recurrence rates mentioned in the articles is difficult to assess because only one author gave confidence intervals of recurrence rates or depression at follow-up.13
Limitations of Our Review
Although we made our choice of inclusion criteria to ensure reasonable comparability, older studies52,53 and those in which data on depression could not be extracted from a broader variety of mental illness in family practice54 were excluded. We also made the choice to describe a limited number of outcomes, but the small number of studies included and the variety within the studies did not allow a review of more outcome results.
Although the calculation of a total recurrence rate may be criticized, we think that the fluctuating nature of depression justifies this procedure.
Conclusions
There are large gaps in the available knowledge about long-term outcome of depression in primary care, and future studies are required to fill in these gaps. We recommend the following:
- The outcome of all types of depression should be evaluated in prospective studies, with a follow-up of at least 5 years, of representative samples in both community and primary care.
- Continuous morbidity registration should be used. With this aim, data meeting fixed criteria that have been established beforehand should be collected longitudinally.
- Studies should include naturalistic follow-up and relate treatment to outcome.
- Quality-of-life assessment should be included.
Recommendations for clinical practice
Family physicians can reassure patients with depression by telling them that although the long-term outcome of the illness is not completely clear, there are indications that the majority of patients with depression do not have a poor prognosis. As the long term risk of recurrence seems to be approximately 40%, most patients in primary care settings only have 1 episode of depression. This information might aid a patient’s recovery.