The Danish National Patient Registry: a review of content, data quality, and research potential

Morten Schmidt, Sigrun Alba Johannesdottir Schmidt, Jakob Lynge Sandegaard, Vera Ehrenstein, Lars Pedersen, Henrik Toft Sørensen, Morten Schmidt, Sigrun Alba Johannesdottir Schmidt, Jakob Lynge Sandegaard, Vera Ehrenstein, Lars Pedersen, Henrik Toft Sørensen

Abstract

Background: The Danish National Patient Registry (DNPR) is one of the world's oldest nationwide hospital registries and is used extensively for research. Many studies have validated algorithms for identifying health events in the DNPR, but the reports are fragmented and no overview exists.

Objectives: To review the content, data quality, and research potential of the DNPR.

Methods: We examined the setting, history, aims, content, and classification systems of the DNPR. We searched PubMed and the Danish Medical Journal to create a bibliography of validation studies. We included also studies that were referenced in retrieved papers or known to us beforehand. Methodological considerations related to DNPR data were reviewed.

Results: During 1977-2012, the DNPR registered 8,085,603 persons, accounting for 7,268,857 inpatient, 5,953,405 outpatient, and 5,097,300 emergency department contacts. The DNPR provides nationwide longitudinal registration of detailed administrative and clinical data. It has recorded information on all patients discharged from Danish nonpsychiatric hospitals since 1977 and on psychiatric inpatients and emergency department and outpatient specialty clinic contacts since 1995. For each patient contact, one primary and optional secondary diagnoses are recorded according to the International Classification of Diseases. The DNPR provides a data source to identify diseases, examinations, certain in-hospital medical treatments, and surgical procedures. Long-term temporal trends in hospitalization and treatment rates can be studied. The positive predictive values of diseases and treatments vary widely (<15%-100%). The DNPR data are linkable at the patient level with data from other Danish administrative registries, clinical registries, randomized controlled trials, population surveys, and epidemiologic field studies - enabling researchers to reconstruct individual life and health trajectories for an entire population.

Conclusion: The DNPR is a valuable tool for epidemiological research. However, both its strengths and limitations must be considered when interpreting research results, and continuous validation of its clinical data is essential.

Keywords: epidemiological methods; medical record linkage; registries; research design; validation studies.

Figures

Figure 1
Figure 1
Timeline for the content and classification systems in the Danish National Patient Registry. Abbreviations: DCECI, Danish Classification of External Causes of Injury; ed, edition; ICD, International Classification of Diseases; NCECI, Nordic Classification of External Causes of Injury; NOMESCO, Nordic Medico-Statistical Committee.
Figure 2
Figure 2
User interface of the Danish Health Care Classification System (SKS browser). Notes: Available at http://www.medinfo.dk/sks/brows.php. English translation (consecutive line order): administrative data/classifications of treatment and care/classification of diseases/classifications of external causes of injury/International Classification of Functioning (ICF)/classification of surgery/Anatomical Therapeutic Chemical (ATC) classification system/anesthesia, intensive care, prehospital care/classification of examinations/temporary classification of examinations/specifications/supplementary codes/ZZ-procedure codes/ZZK, ZZP, ZZV, and ZPP codes and pseudo procedure codes (cancer, cancelled, discontinued)/classification of hospitals and departments. Abbreviation: SKS, Sundhedsvæsenets Klassifikations System.
Figure 3
Figure 3
Timeline for the initiation of selected Danish registries linkable to the Danish National Patient Registry. Abbreviations: ADHD, attention deficit hyperactivity disorder; ICD, International Classification of Diseases; GP, general practitioner; COPD, chronic obstructive pulmonary disease.

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