Different studies vary considerably in their working diagnostic criteria for food allergy. This has an important influence on the resultant measurement of prevalence and incidence in a population. In looking at IgE-mediated allergic problems, there are three levels of diagnostic criteria: (1) questionnaire-based histories, (2) specific IgE and/or skinprick testing and (3) food challenges (see Chapter 3). If, for example, we compare two population studies defining the prevalence of cows' milk allergy, one using skin testing and the other questionnaire-derived data, a higher prevalence will emerge in the latter study design. Double-blinded, placebo-controlled food challenges represent the gold standard but can not be practically used in large population-based studies where a combination of skinprick testing and questionnaire-based histories is more applicable.
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