The Danish unique personal identifier and the Danish Civil Registration System as a tool for research and quality improvement.

All countries want to improve the health of their populations and to improve the quality of care and patient safety. Consequently, there is an ongoing need to assess and document population health, the quality of care and patient safety using valid and reliable data. This requires the ability to monitor the same individuals over time as they receive prevention, diagnostics, treatments, care and rehabilitation and experience improvements or deteriorations in their health or healthcare. This is, however, a challenge for most healthcare systems. A prerequisite to such data is the unique personal identifier. This perspective on quality paper describes the experience with the unique personal identifier in Denmark, based on the Danish Civil Registration System (DCRS) as a tool for research in epidemiology, health services research, quality improvement and patient safety. DCRS has been celebrating its 50 years anniversary.