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The Danish National Patient Register

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The Danish National Patient Register (NPR) was established in 1977, and it is considered to be the finest of its kind internationally. At the onset the register included information on inpatient in somatic wards. The content of the register has gradually been expanded, and since 2007 the register has included information on all patients in Danish hospitals. Although the NPR is overall a sound data source, both the content and the definitions of single variables have changed over time. Changes in the organisation and provision of health services may affect both the type and the completeness of registrations. The NPR is a unique data source. Researchers using the data should carefully consider potential fallacies in the data before drawing conclusions.
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Scandinavian Journal of Public Health
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DOI: 10.1177/1403494811401482
2011 39: 30Scand J Public Health
Elsebeth Lynge, Jakob Lynge Sandegaard and Matejka Rebolj
The Danish National Patient Register
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Scandinavian Journal of Public Health, 2011; 39(Suppl 7): 30–33
DESCRIPTION OF DANISH REGISTERS
The Danish National Patient Register
ELSEBETH LYNGE
1
, JAKOB LYNGE SANDEGAARD
2
& MATEJKA REBOLJ
1
1
Institute of Public Health, University of Copenhagen, Copenhagen, Denmark,
2
National Board of Health, Copenhagen,
Denmark
Abstract
Introduction: The Danish National Patient Register (NPR) was established in 1977, and it is considered to be the finest of its
kind internationally. Content: At the onset the register included information on inpatient in somatic wards. The content of the
register has gradually been expanded, and since 2007 the register has included information on all patients in Danish
hospitals. Validity and coverage: Although the NPR is overall a sound data source, both the content and the definitions of single
variables have changed over time. Changes in the organisation and provision of health services may affect both the type and
the completeness of registrations. Conclusion: The NPR is a unique data source. Researchers using the data should
carefully consider potential fallacies in the data before drawing conclusions.
Key Words: Patient registration, hospital activity registration
Introduction
The Danish National Patient Register (NPR) is a
key Danish health register [1,2]. Established in 1977,
the register has been expanded over the years, from
covering originally only somatic inpatients to cover-
ing today somatic as well as psychiatric in- and out-
patients in all hospitals. The register is also increas-
ingly used as a source register for more specialised
health registers, e.g. the Medical Birth Register since
1995. Internationally, the NPR is considered to be the
most comprehensive of its kind. Although national
hospital registers exist in other Nordic countries, these
registers are less comprehensive than the Danish one.
Finland started a national hospital register in 1970, but
this register covers a smaller proportion of healthcare
activities than the Danish register. The Swedish
national register started in 1987. The Norwegian
register started in 1997 and has had personal identi-
fication numbers from 2009. A nationwide hospital
register has existed in the Netherlands since 1963,
but personal data are anonymous and therefore not
linkable to other registers.
Traditionally, Danish hospitals have been run by
the counties, and the regions since 2007, and
financed via tax revenue. County-based hospital
registration systems were set up in the 1970s, and
this formed the basis for the creation of the NPR in
1977, where the National Board of Health requested
the counties to provide the data from public hospi-
tals. At the onset, the NPR covered inpatients in
somatic wards, as a separate register for inpatients in
psychiatric wards had existed since 1969. From 1995
onwards, all outpatient activities, emergency room
contacts, and activities in psychiatric wards have been
included in the NPR. The upcoming of a private
healthcare sector in Denmark in the 2000s necessi-
tated notification from private hospitals and clinics to
the NPR, which in full (including outpatient con-
tacts) became compulsory in 2003. While the NPR
was originally a monitoring instrument for hospital
activities, it has from 2000 onwards served also as the
basis for the payment of public as well as private
hospitals via the Diagnostic Related Group (DRG)-
system.
Content
The data in the NPR are organised in a structured
way with each variable having a limited number of
codes. The codes can be converted into free text via
Correspondence: Elsebeth Lynge, Institute of Public Health, University of Copenhagen, Copenhagen, Denmark. E-mail: elsebeth@pubhealth.ku.dk
(Accepted 28 January 2011)
ß2011 the Nordic Societies of Public Health
DOI: 10.1177/1403494811401482
translation modules, otherwise the NPR includes
no unstructured information. The data reported
to the NPR can be divided into two types: admin-
istrative data and clinical data. The administra-
tive data include identification of the patient (the
unique personal identification number – CPR-
number), patient’s municipality, identification of
hospital ward, date and time of activity, and infor-
mation on accidents leading to hospital contact. The
clinical data include diagnoses and surgical
procedures.
The basic information reported to the NPR
has been the same since the creation of the regis-
ter in 1977 (Table I). Yet, over time, changes have
been made in variables and classifications, and these
changes must be taken into consideration when
data are to be retrieved from the NPR. No change
was made during the first 10 years. Changes were
made in 1987, followed by 5 years without changes.
Since the early 1990s, changes have been made
almost yearly. The most important changes have
been the gradual expansion of the register.
From 2007 onwards, the NPR has included
all contacts to hospitals in Denmark. A recent strategy
is to trim reported information to minimise the
registration burden for healthcare providers.
The data content of the NPR and the changes
which have taken place over time are documented
in the Danish language publication ‘‘Common con-
tent for basal registration of hospital patients’’,
which is now available in the 19th version [3].
Documentation can be found also on the website
of the National Board of Health (www.sst.dk).
In 1987, registration of type of admission, referral,
and type of discharge was simplified, and more
detailed data on accidents were requested. This
accident registration remained unchanged until
2008, when the latest version of the Nordic
Classification of External Causes of Injury
(NCECI) was introduced.
The next major change took place in 1994, when
Denmark adapted the International Classification
of Diseases, version 10 (ICD-10). Previously, diag-
nostic information was coded according to ICD-8.
In 1995, data were added on outpatients, patients
from emergency rooms, and psychiatric wards. The
classification of surgical procedures was changed in
1996, where the Nordic Classification of Surgical
Procedures (NCSP) was introduced.
The classifications used in the NPR can, with the
exception of a few minor classifications, be found in
the web-based Health Care Classification (SKS)
system, which provides information also on
the period for which a given code has been in use.
The SKS system can be found on www.sst.dk.
For registration of patients in the NPR as well as
for retrieval of patient information from the NPR,
it is of utmost importance to use the correct
SKS codes. This is not easy, as the SKS includes
many codes which might from a clinical point of
view not be mutually exclusive. To facilitate the
search for SKS codes, the National Board of Health
has developed the so-called SKS browser, where it is
possible in the presently valid codes to search for
the correct code via free text in both Danish
and Latin, e.g. for breast cancer ‘‘brystkræft’’ and
‘‘cancer mamma’’. The browser does not at present
cover historical codes, which is a limitation for
retrieval of the historical data needed in research
projects.
Validity and coverage
The validity and the coverage of the NPR data
have been investigated previously [e.g. 4,5]. Here
we will focus on a few aspects only, to which a
potential user of the NPR data should pay particular
attention.
Firstly, in using the NPR data it is important to be
aware of the many definitions used in the system. As
an illustration, in the National Board of Health
publication Hospital statistics 2005 [6], the reported
number of outpatient visits was 6,336,000. In the
National Board of Health data retrieval system
Table I. Key variables in the Danish National Patient Register.
Variable
CPR-number
Hospital department
Date and time for hospital arrival and departure, outpatient contact, treatment, and operation
Waiting status
Referral diagnosis
Action diagnosis
Other diagnoses
Type of operation, examination, and treatment
The Danish National Patient Register 31
‘‘Health data, Hospital data, Hospital activity’’ [7],
the concept of outpatient visits cannot be found,
while numbers are given for outpatient contacts,
being 1,894,441 in 2005. If a cross reference was not
made between the two data sources, one might easily
just use one of them.
Secondly, numbers might not correspond
despite using the same definitions. In Hospital statistics
2005, a total of 1,205,000 hospital discharges and
981,000 emergency room contacts are reported. From
the web-based Health data, these numbers are
1,180,164 and 933,610, respectively. The first dataset
includes psychiatric wards and excludes private hos-
pitals, whereas the second dataset excludes psychiatric
wards but includes private hospitals.
Thirdly, awareness should be given to changes
of codes over time. An example is the very impor-
tant variable ‘‘patient type’’. Codes 0, 1, 2, and 3
have been used for this variable, but the meaning
of the codes has changed over time. Code 2 was thus
used for night patients up until 1987, thereafter it was
used for outpatients (Table II).
Fourthly, as the NPR has since 2000 formed
the basis for payment to public hospitals, the registra-
tion from these hospitals is assumed to be complete
since then. However, although reporting of all activity
from private hospitals and clinics has been mandatory
since 2003, this registration is known to be incom-
plete. In 2008, the National Board of Health esti-
mated that 5% of all operations were missing from the
NPR. Private hospitals offer services paid by taxes
under the rules of ‘‘free hospital choice’’ or as part of
an agreement with a region, and services paid privately
either by insurance companies or private persons.
Especially the latter are expected to be incompletely
registered. The National Board of Health runs infor-
mation campaigns to promote registration, but there is
no validated list of private hospitals and clinics.
Fifthly, the use of the DRG-system for payment
of hospitals and politically decided treatment guar-
anty periods may cause a drift in the coding
of diagnoses and in the use of certain types of
treatments. It has frequently been pointed out that
orthopaedic surgery is often used for patients where
watchful waiting and/or physiotherapy would have
been more relevant. Unfortunately, no study with
numerical documentation of this phenomenon has to
our knowledge been undertaken. This is highly
warranted, as this drift will be inappropriately
reflected in the NPR as a change in the disease
pattern.
Finally, there is at present a lack of consistency
between the NPR data and the data in clinical
databases [8]. For example, according to the web-
based Health data, a total of 22,743 incident cases of
colorectal cancer occurred in Denmark in 2001–
2006 [9]. According to the statistics from the Danish
Colorectal Cancer Group, however, this number was
19,545 [10], a difference of 14%. Linkage of the
NPR data with the clinical databases, e.g. by using
NPR as an in-data provider, would improve the
completeness of the clinical databases.
Conclusion
The NPR is a very important register for biomedical
and public health research. It is, however, a compli-
cated register where many potential fallacies have to
be taken into account in order to ensure correct
retrieval of register data.
Funding
This research received no specific grant from any
funding agency in the public, commercial, or not-for-
profit sectors.
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Danish]. Available at: www.sst.dk/Indberetning%20og%20
Table II. Coding of patient type in the Danish National Patient Register.
Code ‘‘Pattype’ Text in English Text in Danish From date (dd.mm.yyyy) To date (dd.mm.yyyy)
2 Night patient Natpatient 01.01.1977 31.12.1986
1 Day patient Dagpatient 01.01.1977 31.12.1986
0 24 hours patient Heldøgnspatient 01.01.1977 31.12.2001
3 Emergency room patient Skadestuepatient 01.01.1987 Present
2 Out patient clinic patient Ambulatoriepatient 01.01.1987 Present
1<24 hours patient Deldøgnpatient 01.01.1987 31.12.2001
0 Inpatient Indlagt patient 01.01.2002 Present
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The Danish National Patient Register 33
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Dansk Kolorektal Cancer Database The Danish National Patient Register
  • København
København: Dansk Kolorektal Cancer Database, 2008. The Danish National Patient Register
Monitoring of quality based on data in the Danish National Patient Register [in Danish
  • Ø Lisegaard
  • Vestergaard Ch
  • Hammerum
  • Ms
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