Introduction

Pharmacoepidemiology is the study of medication use and its effects in large populations.1 A key objective of pharmacoepidemiological research is post-approval detection of adverse effects not identified in phase 3 trials, achieved by examining medication use in populations treated in routine clinical practice.2 Many medication withdrawals based on phase 4 studies — including early examples such as diethylstilbestrol;3 publicly well-known cases, such as thalidomide4 and rofecoxib;5 and, more recently, ranitidine6 — have led to increased demand for evidence-based drug safety beyond traditional passive surveillance.7

The increased digitization of healthcare has enabled the creation of medical databases containing vast amounts of data over long periods.8 These extensive databases have provided possibilities for conducting low-cost, large-scale, population-based pharmacoepidemiological research on rare and long-term adverse effects, which were impossible to accomplish decades ago, but few databases include detailed data on medication use from patients treated in hospitals.9,10 The databases recording in-hospital medication use do not usually capture all segments of a target population because registration in the databases, except in a few cases,11–13 is often predicated on qualifying factors such as age; employment or insurance status; or hospital health network affiliation or specialty.14

Denmark has established many high-quality health registries over decades.8 One is the Danish National Prescription Registry,15 which holds detailed information on prescriptions dispensed at all Danish community pharmacies and is widely used in pharmacoepidemiology. However, the registry does not record data on in-hospital medication use. The recently established Danish National Hospital Medication Register (DHMR) provides detailed data on medication used within all public hospitals in Denmark. This advancement enhances opportunities for research by allowing for in-depth analysis of medication use in the community and during hospital stays.

Several hospital medications, including particularly potent medications such as chemotherapeutic agents16–18 and reserve antibiotics,19–21 can have severe adverse effects and long-term consequences. Current research on adherence to chemotherapy protocols and consistency with national chemotherapy guidelines,22 personalized dosing in decreasing chemotherapy toxicity,23 reinfection rates after use of broad-spectrum antibiotics, effectiveness and risk of antibiotic de-escalation strategies, or optimal timing for transitioning from intravenous to oral antibiotics24 has been limited by relatively small sample sizes, reliance on medical record reviews, or a complete lack of studies.

Therefore, the DHMR was established after the Finance Act of 2017 to monitor medication utilization and safety, and enhance research opportunities.25 This article provides an overview of the data recorded in the DHMR (in Danish; Sygehusmedicinregisteret) and evaluates its strengths and limitations as a research tool, as any epidemiological study should attempt to reduce random and systematic errors.

Setting

Denmark’s healthcare system provides a foundation for understanding the management of in-hospital medications. This system is universal, tax-funded, and governed by five regions managing healthcare services;26 the Capital Region of Denmark (~1.9 million inhabitants), Region Zealand (~0.8 million inhabitants), the Region of Southern Denmark (~1.2 million inhabitants), the Central Denmark Region (~1.4 million inhabitants), and the North Denmark Region (~0.6 million inhabitants).27 Out-of-pocket expenditures are limited mainly to primary care and outpatient clinic medication and are often partially reimbursed.28 Depending on the setting and the type of medication order, prescription medications can be acquired from community pharmacies, dispensed through hospital outpatient clinics, or administered during hospital admission.29

In 2022, 2.9 million30 of Denmark’s then 5.8 million inhabitants experienced 792,000 hospital admissions lasting longer than 12 hours and had 12.2 million hospital outpatient clinic visits.31 Approximately 1.8% of these contacts were handled in the private sector.32 Medications used at public hospitals and hospital outpatient clinics accounted for 5.7% of all defined daily doses in Denmark, amounting to 206 million daily doses, compared with 3,614 million daily doses sold at community pharmacies.33,34 This uneven distribution is likely driven by many hospital medications not having defined daily doses, whereas the majority of medications sold at community pharmacies do. The expenses for medications used in hospitals and those sold at pharmacies were more similar. Costs for medication used at public hospitals totaled 1,234 million DKK (165 million EUR), half of which was dispensed through hospital outpatient clinics. In comparison, community pharmacies sold medication for 1,390 million DKK (186 million EUR).33,35

In Denmark, in-hospital medication use is documented in a two-step process through electronic medication management systems. These systems, integrated into electronic medical record systems, are used for both medication orders and to document medication administration. First, a physician makes a medication order, with details including the medication type, dosage, administration route, and dosing frequency and duration. Subsequently, when the medication is administered or dispensed, often by a nurse, the time of administration is recorded, and other information is verified or adjusted according to actual administration.

Two electronic medical record systems have been used in Denmark in all public hospitals since 2022: (1) Columna CIS (Systematic A/S, Aarhus, Denmark) in the North Denmark Region, Central Denmark Region, and Region of Southern Denmark, and (2) Epic (Epic Systems Corporation, Verona, WI, USA) in Region Zealand and the Capital Region of Denmark. The processes and systems described above lay the foundation for data recorded in the DHMR.

Content of the Registry Geographic, Clinical Specialty, and Medication Coverage

Since its establishment in May 2018, the DHMR has recorded data on medication administration and dispensing across all approximately 50 public hospitals and associated hospital outpatient clinics. Electronic medication management systems, the primary data sources for data recorded in the DHMR, have not been fully implemented in all types of departments and differ among regions. In some regions, entire clinical specialties, such as radiology, anesthesiology, ophthalmology, and subdivisions of oncology and hematology care, and departments such as neonatal and intensive care units, have been allowed only to use these medication management systems for certain types of medication administration, or have been exempt from implementing them altogether. Therefore, data on medications delivered mostly in these settings, such as cytostatic agents, biological therapies, and anesthetics, are currently not recorded completely in all five regions. However, in the next several years, the Danish Health Data Authority intends to record all medications used in all hospital-related settings in the DHMR.36 A region-based overview of the departments and types of medications currently not reported to the DHMR is presented in Table 1.

Table 1 Overview of Current Reporting Limitations, by Department and Medication Type, to the Danish National Hospital Medication Register Across Danish Administrative Regions

Only data from public hospitals, including emergency departments and hospital outpatient clinics, should be reported by the five regions to the DHMR. Therefore, the regions use the National Catalogue of Health Organisations (in Danish; Sundhedsvæsenets Organisationsregister) codes to exclude data from entities outside public hospitals, such as hospices and the few private hospitals, as defined by the Danish Ministry of Health, as well as medications administered in the prehospital setting.

Denmark uses a national online solution to improve medication reconciliation processes ensuring that prescribed medications are reviewed and aligned with what patients are actually taking. This system, the Shared Medication Record (in Danish; Fælles Medicinkort), enables healthcare providers to share and update patient medication information across healthcare sectors.37 In the DHMR, updates to medications prescribed before a hospital admission are not recorded if hospital personnel only review or adjust prescriptions, or if patients bring medications to the hospital for self-administration. In addition, if a hospital medication order extends into a prescription and is redeemed at a community pharmacy, it is only recorded in the Danish National Prescription Registry.15

All data in the DHMR are collected by the regions and provided to the registry by daily data transfers, and each region is responsible for timely reporting and data validity while the Danish Health Data Authority manages the registry.

Information in the Registry

The registry contains 34 variables, most directly associated with the detailed recording of medication type, order, administration, and dispensing. The registry records administrative patient data, including civil registration numbers and other unique identification codes, enabling accurate record linkage at the individual level for a specific hospital contact and treatment course. The Anatomical Therapeutic Chemical (ATC) classification system is used to specify the medications’ active substances and therapeutic uses.38 Additionally, a six-digit product number, mandatory on all pharmaceutical packaging sold in Denmark, provides package-related information, such as price, pharmaceutical form, strength, and quantity. The Danish Health Data Authority provides a continually updated variable list with detailed descriptions of each variable. A summary of current variables available for research in the DHMR is provided in Table 2.

Table 2 Summary of Variables in the Danish National Hospital Medication Register

From its establishment through December 2023, the DHMR recorded data on more than 1.9 million unique patients with one or more inpatient or hospital outpatient clinic medication administrations or dispensing, amounting to 105.3 million recordings. Table 3 shows the medications most frequently recorded in 2023, according to the number of unique patients with at least one administration or dispensing, and includes the corresponding total administrations, and their distribution by inpatient or outpatient clinic setting. That year, 863,666 unique hospital patients had a combined 32.3 million administrations and dispensing recorded in the DHMR. Similar numbers have been recorded each year since the establishment of the registry in 2018.

Table 3 Fifteen Most Frequent Medications by Number of Unique Patients, Total Administrations, and Administrations According to Inpatient or Outpatient Setting Labels Recorded in the Danish National Hospital Medication Register in 2023

Table 4 presents the annual distribution of medications used among unique patients, categorized according to the main anatomical groups of the ATC system. The distribution across ATC main groups was similar throughout the period. Annually, the approximate percentages of patients receiving at least one administration or dispensing of medications classified under the three largest ATC main groups were as follows: 21%, “nervous system”; 14%, “blood and blood forming organs”; and 13%, “alimentary tract and metabolism”.

Table 4 Annual Distribution of Patients in the Danish National Hospital Medication Register by Count, According to ATC Main Groups

Selected examples illustrating the extent of patients’ exposure to medications used in hospital settings, associated with high costs, or having potential for severe adverse effects are highlighted in Table 5.

Table 5 Total Counts and Percentages of Patients Receiving Selected Medications, Recorded in the Danish National Hospital Medication Register Among 863,666 Unique Patients in 2023

Data Quality

The medication data recorded in the DHMR are generated during clinical care, and data recorded in such contexts often have high validity. However, some medication administrations may not be recorded, which could reduce the sensitivity of assessing medication exposure.

One study evaluating a regional hospital medication registry covering hospital medication use in the Capital Region of Denmark has demonstrated that a high proportion, 100% (95% confidence interval, 98.4%–100%), of 227 medication administrations in the electronic medical record system was also recorded in the registry. For clinical observations, 98.3% (95% confidence interval, 95.1%–99.6%) of 176 medication administrations recorded by investigators observing registered nurses during dayshifts were later found in the regional registry.39 However, the specific electronic medication management systems in use, when data were recorded for that registry, were replaced before the initiation of the DHMR.

During electronic medication ordering, most fields in these systems require mandatory filling and are designed to focus on detailed data for clinical documentation. Therefore, data entry errors are likely to be minimized, data loss is avoided, and standardization in coding is enhanced.

Record Linkage to Other Registries

All individuals residing in Denmark for more than three months are assigned a unique civil registration number,40 which is registered every time an individual is in contact with the healthcare sector. This enables the use of national registries as sampling frames for creating cohorts according to the identification of events such as exposures, clinical outcomes, and other variables. Examples of frequently used registries for record linkage in pharmacoepidemiology include the Danish National Patient Registry,41 the Danish Psychiatric Central Research Register,42 and the Danish Cancer Register,43 as well as administrative registries containing information such as socioeconomic characteristics, eg, the DREAM database with data on social benefits,44 and the Danish Integrated Database for Labour Market Research which records data on labor market participation.45

The application of data privacy protocols, including pseudonymization and encryption of personal data before linkage, is integral to data linkage. The Danish Health Data Authority ensures that data shared across databases remain de-identified.

Strengths and Limitations Strengths

A major strength of the DHMR is its potential to achieve full coverage of nearly all medication administrations in all public hospitals in Denmark, particularly in the future when all regions fully report data across all departments and medication types.

With the possibility of record linkage to other health registries and clinical quality registries and the opportunity for nearly complete follow-up on death and emigration through the Danish Civil Registration System, the DHMR opens new possibilities in pharmacoepidemiology for studying effects and safety monitoring of treatments used in hospitals. Combined with the Danish National Prescription Registry, researchers can create a complete overview of patients’ overall medication use across general practitioners and hospitals. The DHMR also provides new opportunities to examine off-label medication use and guideline compliance.

Double-counting and misrepresentation of medication sources are limited by documentation of only medication orders with subsequent administration or dispensing by hospital staff. Another strength of the DHMR is the fact that actual administration to inpatients can be tracked, thereby eliminating concerns regarding missing data regarding adherence.

Finally, the registry is updated daily, including information on whether earlier reported data have been changed or deleted. Two regions have assessed the proportion of medication administrations linkable to contacts in the Danish National Patient Registry, and both found a linkage in approximately 95% of cases.36 This linkage allows for accurate assessment of the context of medication administration, including detailed disease and patient trajectories.

Limitations

The registry’s recent establishment in May 2018 may cause, in rare cases, a risk of patients’ medication history being left truncated for selected treatments because some patients will have already experienced an adverse event or medication exposure before the data collection began.8,46 This may be addressed by including only incident patients.

Also, studies on small sample sizes of patients treated with rarely used medications may be affected by low precision of the risk. Additionally, the registry does not record data from the private sector, which often involves less complex cases. More than half of the referrals to private hospitals are for brief diagnostic visits, and prolonged inpatient care is almost non-existent. However, the private sector accounts for less than 1.5% of Danish healthcare expenses.

As summarized in Table 1, not all hospital departments use electronic medication management systems, but some departments, including oncology and hematology, partly use these systems and, therefore, only report certain medications as administrations, some as procedures, or both. Another limitation is that not all variables have been collected from the start of the registry, thereby increasing the risk of incomplete longitudinal data.

Because medication packages are scanned in connection with medication administration only in the Capital Region of Denmark and in Region Zealand, the validity of medication documentation and cost assessments might differ between regions. Over time, however, it will be mandatory to report all administrations and new data documentation for the DHMR is scheduled for early 2025.36 It aims to provide a better and more updated overview of the registry’s content. A limitation regarding dispensing is that, as with most prescription registries, the period for which medications are dispensed is often unknown.

Finally, because no studies have yet directly evaluated the data validity of individual variables in the DHMR, some concerns may be raised regarding how accurately the DHMR reflects clinical records and whether the data in the electronic medication management systems accurately represent the medications actually administered to patients.47

Data Access

Access to the DHMR is facilitated through the Danish Health Data Authority’s Research Service48 (Danish, Forskermaskinen), which provides secure remote access to data on its servers. To gain access, researchers must work at or collaborate with a Danish institution authorized by the Danish Health Data Authority because only Danish institutions can take on data responsibility for Danish health data.

Accessing DHMR data involves an application process specifying the population and variables needed and requires prior permission from the Danish Data Protection Agency. This permission ensures safety precautions for data processing and defines cancellation deadlines. Researchers working with register-based data in Denmark must comply with the European Data Protection Directive (2016/679)49 and the Danish Data Protection Act (Act No. 502),50 which governs access to individually identifiable health data for research while protecting privacy rights and promoting quality research.

Perspectives

Establishing the DHMR represents an important advancement in pharmacoepidemiological research, not only in Denmark but also globally. As the DHMR continues to expand to include all departments and clinical specialties across Denmark, the registry has the potential to provide new insights into the safety and effectiveness of hospital medication use across a nationwide cohort. This is further enhanced by linking data to the Danish Civil Registration System, which enables accurate record linkage and provides data on events such as emigration or death, enabling near-complete follow-up.

In Denmark, access to registry data on hospital medication use not explicitly collected for research47 has previously been limited to the Danish National Patient Registry or clinical quality registries. Although valuable, these sources record basic and often incomplete data, and often only regarding specialized treatments in selected patient groups, thus highlighting the need for a more comprehensive data source with broad coverage, such as the DHMR. In addition, the DHMR might aid in post-approval monitoring of medications by providing real-world evidence of their effectiveness and safety. The insights gained from data on treatment adherence, patient outcomes, and side effects, including data from comparative effectiveness studies, can provide evidence of a medication’s effectiveness in everyday clinical practice.

Further, with increasing hospital medication costs and limited opportunities for conducting health economic studies, the detailed data available through the DHMR can support health economics analyses used for prioritizing resources effectively within the healthcare system and supporting the advancement of personalized medicine.

The DHMR may provide medication data regarding cross-sectoral healthcare utilization and medication errors across care transitions, which was previously unavailable when patients transferred between sectors. Moreover, as similar registries may be established in other countries in the coming years, international collaboration may strengthen in-hospital and hospital outpatient clinic pharmacoepidemiological research on conditions with small numbers of patients, such as new medications for rare conditions. As the DHMR evolves, the registry is likely to have the potential to become an integral part of pharmacoepidemiological research in Denmark.

Ethics Statement

The project was registered in the data processing inventory of Aarhus University (2016-051-000001, record number 812) and The Danish Data Protection Agency (2015-57-0002). Danish law does not require further approval or informed consent for register-based studies using administrative data.

Disclosure

The authors report no conflicts of interest. The Department of Clinical Epidemiology, Aarhus University and Aarhus University Hospital, receives funding from various companies in the form of research grants to (and administered by) Aarhus University. None of these grants are related to the present study.

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