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Aims: All five Nordic countries (Denmark, Finland, Iceland, Norway and Sweden) have nationwide registries with similar data structure and validity, as well as personal identity numbers enabling linkage between registries. These resources provide opportunities for medical research that is based on large registry-based cohort studies with long and complete follow-up. This review describes practical aspects, opportunities and challenges encountered when setting up all-Nordic registry-based cohort studies. Methods: Relevant articles describing registries often used for medical research in the Nordic countries were retrieved. Further, our experiences of conducting this type of study, including planning, acquiring permissions, data retrieval and data cleaning and handling, and the possibilities and challenges we have encountered are described. Results: Combining data from the Nordic countries makes it possible to create large and powerful cohorts. The main challenges include obtaining all permissions within each country, usually in the local language, and retrieving the data. These challenges emphasise the importance of having experienced collaborators within each country. Following the acquisition of data, data management requires the understanding of the differences between the variables to be used in the various countries. A concern is the long time required between initiation and completion. Conclusions: Nationwide Nordic registries can be combined into cohorts with high validity and statistical power, but the considerable expertise, workload and time required to complete such cohorts should not be underestimated. [ABSTRACT FROM AUTHOR]

Background and Aim: Swedish health registries are common sources of data for studies on the effects of obesity surgery, and there is a need to assess the quality of data in these registries. The aim of this study was to validate the registration of obesity surgery in the National Patient Registry (NPR) and the Scandinavian Obesity Surgery Registry (SOReg). Method: We randomly selected 962 out of 8501 registrations of obesity surgery in 2011 from the NPR and SOReg. Registered surgical procedures in the NPR and SOReg were compared to the medical records, and concordance was analyzed by calculating positive predictive value (PPV) with 95 % confidence interval (CI). Results: We received 938 (98 %) medical records for manual review. The overall PPV for obesity surgery was high in the NPR (PPV 97.0; 95 % CI 95.6-98.4) and even higher in SOReg (PPV 99.7; 95 % CI 99.3-100). Accuracy was higher for gastric bypass surgery than for other types of obesity surgery. Registrations that were misclassified as obesity surgery ( n = 44) included reoperations due to complications or reconstruction to normal anatomy after previous obesity surgery ( n = 11) and endoscopic procedures ( n = 10). Conclusion: Obesity surgery registrations in the NPR and SOReg have high accuracy and are reliable sources of data to identify patients having undergone obesity surgery. When it is of importance to distinguish between specific surgical procedures, non-gastric bypass surgeries in the NPR should ideally be supplemented with data from other sources. [ABSTRACT FROM AUTHOR]