Objectives: Healthcare registers are invaluable resources for research. Partly overlapping register entries and preliminary diagnoses may introduce bias. We compare various methods to address this issue and provide fully reproducible open-source R scripts. Methods: We used all Finnish healthcare registers 1969-2020, including inpatient, outpatient and primary care. Four distinct models were formulated based on previous reports to identify actual admissions, discharges, and discharge diagnoses. We calculated the annual number of treatment episodes and patients, and the median length of hospital stay (LOS). We compared these metrics to non-processed data. Additionally, we analyzed the lifetime number of individuals with registered mental disorders. Results: Overall, 2 130 468 individuals had a registered medical contact related to mental disorders. After processing, the annual number of inpatient episodes decreased by 5.13-10.41% and LOS increased by up to 3 days (27.27%) in years 2011-2020. The number of individuals with lifetime diagnoses reduced by more than 1 percent point (pp) in two categories: schizophrenia spectrum (3.69-3.81pp) and organic mental disorders (1.2-1.27pp). Conclusions: The methods employed in pre-processing register data significantly impact the number of treatment episodes and LOS. Regarding lifetime incidence of mental disorders, schizophrenia spectrum disorders require a particular focus on data pre-processing.

The authors have declared no competing interest.

Kimmo Suokas was supported by the Jalmari and Rauha Ahokas Foundation and the Finnish Psychiatric Association. Christian Hakulinen was supported by the Academy of Finland (354237) and the European Union (ERC, MENTALNET, 101040247). Views and opinions expressed are however those of the authors only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them.

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The Research Ethics Committee of the Finnish Institute for Health and Welfare approved the study protocol. Informed consent is not required for register-based studies in Finland.

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

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I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

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I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

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The data that support the findings of this study are available from the National Institute of Health and Welfare (www.thl.fi) and Statistics Finland (www.stat.fi). Restrictions apply to the availability of these data, which were used under license for this study. Inquiries about secure access to data should be directed to data permit authority Findata (findata.fi/en). The method described in this article has been made publicly available and contain supplementary description of each step of the process (https://github.com/kmmsks/hilmo_identify_episodes/).

https://github.com/kmmsks/hilmo_identify_episodes/