Danish nationwide registers for public health and health-related research

Aims: The Nordic countries have a strong tradition of using nationwide social and health registers for research purposes. The aim of the current paper is to provide an overview of the Danish population-based registers in public health and health-related research, and to discuss their strengths and limitations. Methods: Danish registers on somatic and psychiatric hospital contacts as well as care provided by general practitioners were reviewed. The availability of demographic, individual-level variables of relevance for health-related research was summarized. Results: Since 1968, every person living in Denmark has a unique identifier. This identifier is listed in Danish registers enabling linkage of information from a range of registers on an individual level. The nationwide coverage of all patient contacts at somatic and psychiatric hospitals, consultations with general practitioners, purchases of prescribed medications, and a complete follow-up with respect to causes of death support public health studies surveying trends of prevalence and incidence. Historical data on psychiatric and somatic hospitalizations since 1969 and 1977, respectively, allow an in-depth assessment of the burden of disease and time trends. Demographic characteristics of individuals and family units, together with information on education, employment, income, place of residence and migration, are provided by social registers. Conclusions: Register data are fully representative of the entire population with no loss to follow-up. Nationwide coverage also ensures a large sample to investigate events and conditions with low base rates. Clinical observations are limited and often only available for select patient populations. However, the opportunities available for public health research through linkage of register data with the increasing number of nationwide clinical databases, bio-banks and surveys entail promising perspectives for future research.