Registry Linkage

Denmark's nationwide administrative and clinical registries allow follow-up through linkage. Registry linkage partly compensates for the lack of direct longitudinal follow-up and enables long-term studies. DRKS listed matching EUCTR TRNs in all DRKS–EUCTR pairs, while EUCTR listed matching DRKS TRNs in only a minority. ClinicalTrials.gov more often listed matching EUCTR TRNs than EUCTR listed matching ClinicalTrials.gov TRNs in ClinicalTrials.gov–EUCTR pairs. Most potential cross-registrations were linked by a TRN in at least one registry. Bidirectional TRN linkage was uncommon among potential cross-registrations. A unique civil registration number enables ODIN data to be connected with registry information. Some potential cross-registrations were detected through title matching or TRNs in publications rather than registry TRNs. Unidirectional registry links reduce automated discoverability of cross-registrations. Potential registry-linked information includes comorbidities, medication use, laboratory results, hospital diagnoses, medication trajectories, and mortality.