Letters to the Editor
Canadian Depression Prevalence
Dear Editor: On the basis of their review of the epidemiology of depression, Waraich and others (1) suggest that health planners may need to revise down the commonly reported prevalence rates of mood disorders. However, we urge caution before assuming this to be the case. That is because most of the studies reviewed used instruments that have the potential to overlook many cases of depression.
Consider the data used for the review of 1-year prevalence rates, for which the pooled rate for major depression was 4.1%. Of 11 studies reported, 5 used the Composite International Diagnostic Interview (CIDI) (2), and a further 5 used the Diagnostic Interview Schedule (DIS) (3), from which the CIDI was derived. It is important to note that these instruments have exclusion criteria that are rarely specified in the published results.
For example, they have several probe or stem questions that determine presumed clinical significance. Thus they exclude persons whose symptoms are considered to be due to medication, drugs or alcohol, physical illness or injury; those who consider their symptoms to be trivial or who have not consulted a doctor; those who consider that their symptoms do not interfere “a lot” (determined by respondent) with their daily life and activities; and also those who have not taken medication for their symptoms on more than one occasion.
The validity of these exclusions warrants further consideration. We acknowledge that excluding those whose depressive disorder is associated with alcohol and (or) drugs or with concomitant physical illness and injury is consistent with DSM-IV guidelines, but we agree with Paykel that the DSM-IV “assigns separate unjustified categories of medical and substance-induced mood disorders” (4, p 95) At the very least, excluding persons with such comorbidity, which is commonly seen in clinical practice, would result in an appreciable underestimate of depression. In this regard, it is of interest that the CIDI even excludes pregnancy as a physical condition that can cause symptoms, although it is reassuring that the probe guidelines acknowledge that pregnancy is not a physical illness!
The exclusion of those who consider their symptoms to be trivial risks the omission of those who tend to deny the significance of their symptomatology and who have poor mental health literacy. Indeed, data exist demonstrating that the mental health literacy of those in the community who have major depression is no more conducive to identifying depression and recommending its treatment than it is for those without depression (5); therefore, to exclude those who believe their symptoms are trivial is not necessarily a strategy based on existing evidence.
Excluding those who have sought treatment but who have not taken medication more than once may also lead to underestimates of depression prevalence. Poor mental health literacy and the presence of side effects that may mitigate against medication use are but 2 reasons why this criterion might exclude those with depression.
We doubt whether many researchers, let alone the average clinician or health planner, are aware of this potential for both the CIDI and the DIS to underestimate the prevalence of depression and its burden on the community (6).
References
1. Waraich P, Goldner EM, Somers JM, Hsu L. Prevalence and incidence studies of mood disorders: a systematic review of the literature. Can J Psychiatry 2004;49:124–38.
2. World Health Organisation. Composite International Diagnostic Interview. Version 2.1 Geneva: WHO; 1996.
3. Robins L, Cottler L, Keating S. NIMH Diagnostic Interview Schedule Version III Revised. St Louis (MO): Washington University School of Medicine; 1989.
4. Paykel ES. Mood disorders: review of current diagnostic systems. Psychopath 2002;35:94–9.
5. Goldney RD, Fisher LJ, Wilson,DH. Mental health literacy: an impediment to the optimum treatment of major depression in the community. J Affect Disord 2001;64:277–84.
6. Goldney R, Hawthorne G, Fisher L. Is the Australian National Survey of Mental Health a reliable guide for health planners? A methodological note on the prevalence of depression. Aust N Z J Psychiatry 2004;38:635–8.
Robert D Goldney, MD, FRCPysch, FRANZCP
Laura J Fisher, BA
Adelaide, South Australia
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