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Researchers map out the advantages of the Danish CPR system

The Danish civil registration register (CPR register) is a goldmine for health research, but its potential has not been fully utilised. That is the opinion of researchers from Aarhus who are the first people to map out how the Danish CPR register system can be utilised in health science research.

Is there a connection between a hip replacement and a thrombosis in the leg? Do children delivered by caesarean have a greater risk of getting cancer than other children? And are you more exposed to blood clots if your mother or father died from a blood clot? These are some of the questions which epidemiological research examines.

A group of researchers comprising clinical epidemiologists and biostatisticians from Aarhus University and Aarhus University Hospital have now mapped out the different methods to utilise Danish CPR numbers and to obtain new knowledge which will benefit the healthcare sector:

"Denmark is quite unique in having a system like the CPR register that follows citizens from cradle to grave. The majority of doctors and researchers know that the various registers can be linked together by means of the CPR numbers. But many of them are unaware of the register’s full potential. So we have therefore mapped everything out and described the different possibilities," says Medical Doctor and PhD student Morten Schmidt from the Department of Clinical Epidemiology at Aarhus University Hospital and Aarhus University.

Their article has just been published in the prestigious scientific journal European Journal of Epidemiology with an editorial to match.

9.5 million CPR numbers

All Danes have a statutory civil registration number, known as a CPR number. This means that data is available all the way back to 2 April 1968, which is when the register was created. The register, which is updated daily, currently comprises approximately 9.5 million unique CPR numbers assigned to citizens in Denmark and Greenland, of which 5.7 million CPR numbers are active.

In the article, the researchers describe seven different methods to utilise the comprehensive CPR register in health science research. For example the well-known option of linking different registers together to predict a patient’s risk of contracting certain diseases. Cost is another advantage:

"In terms of randomised clinical studies, known as random sample studies, the utilisation of the registers is a cost-effective method that provides the opportunity to follow a large cohort of patients for many years," says Morten Schmidt. 

Huge potential

According to co-author Henrik Toft Sørensen, who is professor of clinical epidemiology and director of the Department of Clinical Epidemiology at Aarhus University, the possibility of using the CPR register to identify persons for large-scale questionnaires with an even distribution of respondents is a good example of how clinical epidemiology research can contribute to improving public health and the healthcare sector. For example in the major national "How are you?” study.

Henrik Toft Sørensen emphasises that data must of course be handled confidentially and anonymously in accordance with the Danish Act on Processing of Personal Data. However, in general terms he believes that the opportunity of having unique knowledge that benefits patients far overshadows the risk of data being abused.

Use of the Civil Registration Number (CPR) register as a research tool


The article describes how the CPR register can be used to:

  1. Connect registers at individual level
  2. Follow patients over a period of many years
  3. Identify death as a terminal point in surveys
  4. Construct control groups from the control population in cohort studies
  5. Construct control groups in case-control studies
  6. Construct family cohorts for the study of familial disease risks
  7. Select random persons for population studies.

Further information


Download the abstract of the original article ”The Danish Civil Registration System as a tool in epidemiology”



Professor, Consultant, MD, PhD Henrik Toft Sørensen

Aarhus University, Department of Clinical Medicine, Department of Clinical Epidemiology 

Email: hts@clin.au.dk

Tel: +45 8716 8215 / +45 5168 0555