Vicente et al (Reference Vicente, Kohn and Rioseco2004) and Weich & Araya (Reference Weich and Araya2004) have made important observations regarding the reporting of substantially different rates of mental disorders, particularly major depression, in two well-designed studies in Chile. The lower prevalence of major depression of 3.4% was determined by using the Composite International Diagnostic Interview (CIDI), and Vicente et al noted that diagnoses were based on an algorithm. However, they did not describe the nature of the exclusion criteria for the diagnosis of major depression contained within that. They are perhaps unexpected, and may at least partly explain the different results.

The CIDI has a number of probe or stem questions that determine the presumed clinical significance, thereby excluding a number of conditions. For example, it excludes those persons whose symptoms were considered to be due to medication, drugs or alcohol, physical illness or injury; those who considered their symptoms to be trivial or who had not consulted a doctor; those who considered that their symptoms did not interfere ‘a lot’ (determined by the respondent) with their everyday life and activity; and those who had not taken medication for their symptoms on more than one occasion.

The validity of these exclusions warrants further consideration. It is acknowledged that the exclusion of 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 (Reference Paykel2002) that the DSM–IV ‘assigns separate unjustified categories of medical and substance-induced mood disorders’. At the very least the exclusion of persons with such comorbidity, which is common 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 considered 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, there are data that have demonstrated 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 in those without depression (Reference Goldney, Fisher and WilsonGoldney et al, 2001). Therefore, the exclusion of those who believe their symptoms are trivial is not necessarily supported by existing evidence.

Exclusion of those who sought treatment but who had not taken medication more than once is also liable to underestimate the prevalence of depression. Poor mental health literacy and the presence of side-effects which may militate against medication use are but two reasons why those with major depression would be excluded by this criterion.

Each of these exclusion criteria is open to interpretation and we doubt whether many researchers, let alone the average clinician, would be aware of this potential for the CIDI to underestimate the prevalence of depression. Weich & Araya noted correctly that prevalence surveys were designed to provide data for local health planners, but Vicente et al observed that planners may well distrust studies when there are marked differences in results.

We have expressed concern about the use of CIDI-derived prevalence figures for depression in Australia, as they could underestimate by at least half both the financial burden on the community and potential service requirements (Reference Goldney, Hawthorne and FisherGoldney et al, 2004). It is probable that these exclusion criteria explain the majority of the difference in the results of the two Chilean studies. We trust that health planners in Chile and elsewhere are aware of the potential for underestimation of depression in studies using the CIDI.

Declaration of interest

R.D.G. has received honoraria, been on advisory boards, and has received. grants from Bristol-Myers Squibb, Janssen-Cilag, Lundbeck, Organon, Pfizer. Australia, Sanofi Synthélabo and Wyeth Australia. G.H. received. financial support from Pfizer Australia, Bristol-Myers Squibb Australia and. Wyeth Australia.