# Diagnoses of Multiple Sclerosis and Related Disorders and Disease‐Modifying Therapies: A Comparison of the Danish Multiple Sclerosis Registry With Other Danish Health Registries

## TL;DR

This study compares data on multiple sclerosis diagnoses and treatments from three Danish health registries to help researchers choose the best data sources for their studies.

## Contribution

The study provides a detailed comparison of MS-related data across registries, highlighting their strengths and limitations for research.

## Key findings

- Most MS patients in DMSR were also recorded in DNPR, but many had unspecified disease phenotypes in DNPR.
- Only 39.8% of DMSR-registered disease-modifying therapies were found in DNHMR, with significant variation by treatment type.
- DMSR is best for detailed MS research, while DNPR is better for broad epidemiological studies.

## Abstract

Comparison of recorded diagnoses of multiple sclerosis (MS) and related disorders and disease‐modifying therapies (DMTs) in The Danish Multiple Sclerosis Registry (DMSR) with other nationwide health registries. The aim of the study is to describe and compare information on diagnoses of MS and related disorders and treatments with DMTs available in three national registries, highlighting the key differences relevant to MS research and providing insight for researchers for their choice of data source(s) suitable for their study.

DMSR is a disease registry encompassing information on persons with MS and related disorders. The Danish National Patient Registry (DNPR) is a registry of activities at Danish hospitals. The Danish National Hospital Medication Registry (DNHMR) contains information on in‐hospital prescription medications. The population comprised all persons in DMSR in 2023 who were alive or born after and residing in Denmark on January 1, 1995 (N = 26,474). For this population, we identified DNPR contacts with diagnoses of MS or related disorders and initiated DMTs in DNHMR. Diagnostic and demographic characteristics were reported as recorded in DMSR for the total population, and the subset included in DMSR but not in DNPR. Characteristics of the part of the population identified in DNPR were reported as recorded in DNPR. We calculated the proportions of DMT treatments in DMSR identified in DNHMR.

Of the 26,474 persons, 23,857 (90.1%) were recorded with a diagnosis of MS or a related disorder in DNPR. Most (86.6%) of the 2617 persons not identified in DNPR were diagnosed before 1995. The proportion of persons with MS recorded without specification of the disease phenotype was 23.5% in the DMSR and 76.2% in the DNPR. After 2005, only 1.8% of persons with MS were recorded with an unspecified phenotype in DMSR. Of a total of 18,168 initiated DMT treatments in DMSR, 7230 (39.8%) were identified in DNHMR, with proportions ranging from 0.0% (mitoxantrone) to 88.5% (ocrelizumab).

DMSR is suitable for disease‐specific research addressing treatment efficacy, disease development, and long‐term outcomes. In contrast, DNPR is well suited for broad epidemiological studies involving various health conditions and hospital utilization. The DNHMR will, when matured, be useful for studies involving medical treatment and comedication. However, as each registry has limitations, for most studies the best approach will be combining registries, depending on the research question.

This study aimed to describe and compare information on diagnoses of MS and related disorders as well as treatments with disease‐modifying therapies (DMTs) available in three national registries, highlighting the key differences relevant to MS research. The Danish Multiple Sclerosis Registry (DMSR) is a disease registry encompassing a wide range of information on persons with MS and related disorders established in 1956. 26,474 people with MS or related disorders, 24,552 people with MS, 76.5% with specified phenotype, 18,168 treatments involving 11,602 people with MS, The Danish National Patient Registry (DNPR) is a registry of activities at Danish hospitals established in 1977. 87.7% of the people with MS or related disorders in DMSR were identified with the same primary diagnosis, 92.4% of the people with MS in DMSR were identified with an MS diagnosis, and 23.8% with a specified phenotype. The Danish National Hospital Medication Registry (DNHMR) contains information on in‐hospital prescription medications established in 2018. 7230 treatments involving 5944 persons (39.8%) were identified compared to DMSR.