In 2019, the Danish National Patient Register was updated, making it no longer possible to directly determine whether a hospital contact concerned an inpatient or an outpatient. To address this challenge, the Data Powerhouse developed an algorithm capable of classifying patient types. Additionally, we are part of a collaborative group aiming to establish national consensus in this area.
Publications
Classification of Inpatient and Outpatient Contacts in the Danish National Patient Register: Epidemiological Implications and Future Directions, Buchard AS, Bodilsen J, Dreyer LW, Duch KS, Magnussen B, Mottlau RG, Møller JJ, Skjødt F, Willumsen M, Jensen A (2026)
With the transition to the third version of the Danish National Patient Register (DNPR3) in 2019, the variable identifying inpatient versus outpatient contacts was removed from the register. For register-based research, this means that researchers must classify hospital contacts as inpatient or outpatient themselves. Several different methods are currently used, and the choice of method may affect study populations, results, and comparability across studies and over time.
In this study, conducted as a national collaboration, we present and discuss the existing knowledge on the classification of hospital contacts in DNPR3 and the associated methodological and epidemiological implications. We highlight the need for a common, transparent, and validated national framework for classifying hospital contacts, developed collaboratively by clinicians, epidemiologists, statisticians, and other researchers working with the Danish National Patient Register.
Data-Driven Algorithms for Classification of In- and Outpatients in the Danish National Patient Register. Buchardt AS, Madsen P, Jensen A (2025)
With the introduction of the latest version of the Danish National Patient Register (DNPR3) in 2019, it was no longer possible for researchers to identify whether a hospital contact concerned an inpatient or an outpatient (the so-called patient type). This study aimed to develop and evaluate algorithms that can categorize a hospital contact as either an admission, an elective outpatient visit, or an emergency outpatient visit (the latter previously known as emergency room visits). The goal was to promote consensus in data interpretation among researchers using Danish health registers.
The study showed that both methods based on departments as well as those using information about the individual hospital contact (e.g., duration and overnight stay), including hybrid approaches that combine both levels, could effectively categorize patient types. The choice of the best approach depends, among other factors, on whether a study has access to data at the department level and whether the study covers both the period before and after the introduction of DNPR3.