Determinants of Illness Severity and Mortality Risk Among Hospitalized Patients With Opioid Use Disorder: A National Analysis, 2015–2022

Objectives: Illness severity may influence the relationship between opioid use disorder (OUD) and adverse outcomes, yet factors affecting severity are not fully understood. The objective of this study is to examine sociodemographic and hospital-related factors associated with severity among OUD-related hospitalizations. Methods: We used data from the Healthcare Cost and Utilization Project (HCUP) National Inpatient Sample (NIS) from 2015 to 2022. The HCUP NIS includes severity measures that assign patients overall severity of illness (SOI) and risk of mortality (ROM) subclasses (ie, minor, moderate, major, and extreme) based on their secondary diagnoses and other clinical characteristics (eg, age, procedures). Partial proportional odds models were used to measure the association between sociodemographic/clinical factors and SOI/ROM subclass severity among OUD-related hospitalizations. Results: SOI shifted toward more severe subclasses in 2019–2022 and ROM in 2015–2022 (higher share major/extreme). Medicare and self-pay, compared with private insurance, were associated with higher odds (adjusted odds ratio aOR 95% CI) of extreme SOI (vs. minor, moderate, or major) (Medicare: 1.07 1.05, 1.09; self-pay: 1.08 1.05, 1.11). Rural hospitals, compared with urban ones, had lower odds of higher SOI severity (aOR 0.80 0.77, 0.82). Results were similar for ROM. Age, sex, race, income, region, hospital size, admission timing, and admission type were also significantly associated with SOI/ROM. Conclusions: Structural/systematic factors play a role in shaping the course of inpatient OUD. These findings highlight the need to strengthen hospital addiction care capacity, address insurance and income-related inequities, and develop targeted inpatient risk stratification strategies to improve outcomes for patients with OUD.