Aim: To describe NHSR in relation to research. Content: The data in the register from general practice is generated through the GPs’
electronic invoices to the Regional Health Administration. Data from 1990 onwards is available covering more than 600 million patient
contacts. For each service the following is registered: 1. The citizen: The unique personal identification number and the allocation to a
general practice. The number make it possible to register the age and sex of the citizen, follow the individual contacts with health care
over time and merge with other registers. 2. The provider: Identification number of the practice, referral to specialist and fees. 3. The
service: Consultations, telephone consultations, home visits, e-mail consultations and preventive consultations. Also laboratory and
additional services like anti-conception counselling are registered. The clinical information is small or absent. The possibilities for linking to
other registers containing health and social information or with ad hoc collected data make NHSR an interesting research tool. Validity and
coverage: All citizens registers with the personal identification number when contacting the health care system. This adds accuracy to the
identification of the patient. There is an economic incentive for the providers to register services as their reimbursement depends on the
invoice. The frequency of errors in the coding of specific services by the provider is not known as no validity studies have been reported.
The data is easy accessible from the Danish National Board of Health. Conclusion: The register's strengths include completeness, size and
long follow-up period. It is useful for research purposes especially when linked with other registers. However, grave reservations must be
made regarding the validity of the register as no studies of this point have been made. Knowledge of the working conditions in general
practice is necessary in order to use the NHSR for research purposes.