Using the Danish Administrative Registers to Define Mental Disorders: Potential and Limitations of Using Different Data Sources

BACKGROUND: Although the Danish health registers provide rich data for epidemiological mental health research, the comparability of findings is hindered by different definitions of mental disorders across studies. We aimed to describe different approaches taken by researchers to define mental disorders using Danish register data and quantify the impact of selecting different definitions on estimates of prevalence and incidence. METHOD: We undertook a scoping review to identify the approaches to define mental disorders using Danish register data and produced a narrative synthesis. We compiled tables of potential definitions for any mental disorder and specific types of mental disorders using nationwide Danish registers. Using these definitions, we quantified (i) the number of individuals fulfilling each definition during 2018 and 2018-2022 and (ii) the estimated cumulative incidence. RESULTS: Our search identified 2185 papers, 1155 of which used Danish register data to define mental disorders to describe the study population, an exposure, or an outcome. Our estimates of the number of individuals fulfilling each definition and the estimated cumulative incidences indicated considerable variation with the data sources and variables used to define mental disorders within the Danish national registers. For example, the cumulative incidence of any mental disorder at 80 years among females varied between 29.2% when only psychiatric unit patient register data were used and 62.1% when somatic patient, prescription, cause of death, and health services register data were added. CONCLUSION: The different data sources and variables used to define mental disorders can result in substantial variation in the numbers of identified cases. Studies should clearly document all variables and values used to define their disorders of interest, as well as describe the disorder they aim to capture, discussing the strengths and limitations of their approach and the likely impacts on the sensitivity and specificity of including or omitting data from other available registers.