Résumé :
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[BDSP. Notice produite par INIST-CNRS jsR0xH8G. Diffusion soumise à autorisation]. There is a need to better define phenotypes of asthma. However, many studies have data available only on asthma and atopy, so they are often used to define'atopic'and'non-atopic'asthma. We discuss and illustrate the problems of analyzing such outcomes. We used the 31 year follow-up of the Northern Finland Birth Cohort 1966 (n=5,429).'Atopic asthma'and'non-atopic asthma'were defined based on presence or absence of atopy (any skin prick test>=3 mm) at age 31. Gender and ownership of cat in childhood were used as risk factors. Simple calculations on hypothetical datasets were used to support the conclusions.'Atopic asthma'and'non-atopic asthma'are not well separated disease entities. The association of a risk factor with'atopic asthma'and'non-atopic asthma'is determined both by its association with asthma and with atopy.E.g. if a risk factor is not associated with asthma, but is protective for atopy, this will produce a protective association with'atopic asthma'but an opposite association with'non-atopic asthma'This is the result from the typical analysis, which uses all non-asthmatics as the comparison group. Valid results, unconfounded by atopy, can be gained by comparing asthmatics to non-asthmatics separately among atopics and non-atopics, i.e. by doing the analysis stratified by atopy. If data only on asthma and atopy are available, asthma and atopy should be analyzed at first as separate outcomes. If atopic and non-atopic asthma are used as additional outcomes, valid results can be gained by stratifying the analysis by atopy.
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