Integrating Clinical and Epidemiologic Data on Allergic Diseases Across Birth Cohorts: A Harmonization Study in the Mechanisms of the Development of Allergy Project.

The numbers of international collaborations among birth cohort studiesdesigned to better understand asthma and allergies have increased in the last several years. However, differences in definitionsand methods preclude direct pooling of original data on individual participants. As part of the Mechanisms of the Development of Allergy ( MeDALL) Project, we harmonized data from 14 birth cohort studies(each with 3-20 follow-up periods) carried out in 9 European countries during 1990-1998 or 2003-2009. The harmonization process followed 6 steps: 1) organization of the harmonization panel; 2) identification of variables relevant to MeDALLobjectives (candidate variables); 3) proposal of a definitionfor each candidate variable (reference definition); 4) assessment of the compatibility of each cohortvariable with its reference definition(inferential equivalence) and classification of this inferential equivalence as complete, partial, or impossible; 5) convocation of a workshop to agree on the reference definitionsand classifications of inferential equivalence; and 6) preparation and delivery of data through a knowledge management portal. We agreed on 137 reference definitions. The inferential equivalence of 3,551 cohortvariables to their corresponding reference definitionswas classified as complete, partial, and impossible for 70%, 15%, and 15% of the variables, respectively. A harmonized database was delivered to MeDALLinvestigators. In asthma and allergy birth cohorts, the harmonization of data for pooled analyses is feasible, and high inferential comparability may be achieved. The MeDALLharmonization approach can be used in other collaborative projects.