Abstract

Objectives
Adjustment for comorbidity is essential in observational studies using administrative data to ensure the reproducibility and transparency of research findings. However, the reporting quality of comorbidity adjustment in studies using the National Health Insurance Claim Data (NHICD) has not yet been evaluated. This study aimed to assess the reporting quality of comorbidity adjustment in health outcome studies that used the NHICD.
Methods
We conducted a PubMed search in April 2025 using the terms “(Korea OR Korean) AND (‘health insurance claim*’ OR claims data OR NHIS OR HIRA) AND (2024).” Of the 239 retrieved studies, 82 outcome studies that exclusively used the NHICD and adjusted for comorbidities as confounding variables were included. Reporting quality was evaluated across 3 domains: (1) scope of data, (2) range of diagnostic codes, and (3) length of the look-back period.
Results
Among the 82 studies, 33 (40.2%) used ad hoc selection, 33 (40.2%) used the Charlson comorbidity index, and 13 (15.9%) applied both methods. The Elixhauser comorbidity measure was rarely used, appearing in only 3 studies. Only 3 studies explicitly described the scope of data used, and 6 reported the diagnostic code range. The look-back period was specified in 26 studies (31.7%), with 1 year being the most commonly reported duration.
Conclusions
The reporting quality of comorbidity adjustment in studies using the NHICD remains suboptimal. Transparent and standardized reporting of data scope, diagnostic code range, and look-back period is essential to improve the reproducibility and comparability of research findings.

• Keywords: National Health Insurance Claim Database; Comorbidity; Confounder; Korea