A FHIR Specification to Formalize Cohort Definitions.

Retrospective studies play an important role in advancing medical research, yet especially cohort definitions are often provided in unstructured text form or in a non-standardized format. This lack of formalization hinders reproducibility, consistency, and automated reuse. Therefore, we present a framework for the structured and standardized specification of cohort definitions within FHIR resources. Drawing from a systematic review of retrospective studies, we derived six modelling categories for cohort definitions (1) patient demographics, (2) standardized medical terminology, (3) clinical results definition, (4) temporal data representation, (5) temporal relationships and dependencies, and (6) logical combination of criteria. Each category is implemented using native or minimally extended FHIR properties, establishing a one-to-one correspondence between cohort definitions and the original clinical data. This enables both human readability and automated processing, supporting use cases such as feasibility searches and transparent cohort documentation in study publications.