Number of ICD-10 diagnosis fields required to capture sepsis in administrative data and truncation bias: A nationwide prospective registry study

Background In observational studies that uses administrative data, it is essential to report technical details such as the number of International Classification of Disease (ICD) coding fields extracted. This information is crucial for ensuring comparability between studies and for avoiding truncation bias in estimates, particularly for complex conditions like sepsis. Specific sepsis codes (explicit sepsis) is suggested identified by extracting 15 diagnosis fields, while for implicit sepsis, comprising an infection code combined with an acute organ failure, the number of diagnosis field remains unknown. Objective The objective was to explore the necessary number of diagnosis fields to capture explicit and implicit sepsis. Materials and methods We conducted a study utilizing The Norwegian Patient Register (NPR), which encompasses all medical ICD-10 codes from specialized health services in Norway. Data was extracted for all adult patients with hospital admissions registered under explicit and implicit sepsis codes from all Norwegian hospitals between 2008 through 2021. Results In 317,705 sepsis admissions, we observed that 105,499 ICD-10 codes were identified for explicit sepsis, while implicit sepsis was identified through 270,346 codes for infection in combination with 240,586 codes for acute organ failure. Through our analysis, we found that 55.3%, 37.0%, and 10.0% of the explicit, infection, and acute organ failure codes, respectively, were documented as the main diagnosis. The proportion of explicit and infection codes peaked in main diagnosis field, while for acute organ failure codes this was true in the third diagnosis field. Notably, the cumulative proportion reached 99% in diagnosis field 11 for explicit codes and in diagnosis field 14 for implicit codes. Conclusion Expanding the utilization of multiple diagnosis fields can enhance the comparability of data in epidemiological studies, both internationally and within countries. To make truncation bias visible, reporting guidelines should specify the number of diagnosis fields when extracting ICD-10 codes.