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Instituto de Saúde Coletiva, Universidade Federal da Bahia, Bahia, Brazil, London, School of Hygiene and Tropical Medicine, London, UK.
To the Editors:
In an article recently published in the European Respiratory Journal, Michel et al. 1 presented the results of a study assessing the accuracy of the response to a question on wheezing in the last 12 months, by comparing responses with parental understanding on wheezing, and building three different scenarios (A, B and C). In the study, Michel et al. 1 accurately raised methodological limitations on estimates of asthma prevalence rates based on such a question and, hence, questionnaires. Although we subscribe to the limitations raised in the paper, we would like to make comments on three statements posed by the authors.
First, the authors stated that "questionnaire surveys are, therefore, more likely to underestimate the true prevalence of wheeze, not the converse". Indeed, all the prevalence rates observed by the authors in the total study population and in subgroups were lower than the rates estimated in scenario B, in which children whose parents had not reported wheeze and given an incorrect definition of wheeze were considered as having asthma. However, this scenario seems to be an extreme situation. If observed rates are compared with scenario C, which the authors describe as "realistic" and maybe closer to the true rate, in only two subgroups (South Asian and the most deprived) would the observed prevalence rates underestimate the true values. It is worth noting that the estimated rates in the three different scenarios are a function of the observed rates. For instance, if the observed prevalence was 20% in both these subgroups and the proportion of correct description among those who reported wheeze and among those who did not report wheeze were as in the study by Michel et al. 1, the estimates in scenario C would be very similar to the observed rates of 20%; that is, there would not be underestimation (table 1
). Estimates of 
20% have been observed in International Study of Asthma and Allergies in Childhood (ISAAC) surveys in several settings in Latin America and in English-speaking countries 2, 3. In conclusion, it seems to us that the statement saying there would be an underestimation of the true prevalence observed in surveys can not be taken as a rule.
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Thirdly, the authors suggest that the presence of atopy should be used to validate the diagnosis of asthma. Indeed, atopy is strongly associated with asthma and, thus, asthma cases with atopy would have a more accurate definition. However, asthma is a syndrome with different aetiologies 4 and defining asthma according to atopy selects a specific phenotype, atopic asthma. Instead of using atopy to increase the validity of the diagnosis, prevalence rates in different subpopulations should be shown separately for atopic and nonatopic asthma.
REFERENCES
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