Association of inpatient outcomes and resource utilization with an ICD-10–defined immune-related adverse event phenotype during U.S. cancer hospitalizations: National Inpatient Sample, 2018–2022.

e23104 Background: Immune checkpoint inhibitor use is rapidly expanding, yet immune-related adverse events (irAEs) are not routinely characterized as an inpatient administrative phenotype with system-level implications during cancer hospitalizations. Methods: A serial cross-sectional, hospitalization-level analysis was performed using the 2018–2022 Healthcare Cost and Utilization Project National Inpatient Sample (NIS). Adult hospitalizations with a principal malignancy were identified using ICD-10-CM codes (C00–C97; D45–D47). An irAE phenotype was defined using any-diagnosis ICD-10-CM codes corresponding to immune-mediated toxicities, including colitis, hepatitis, pneumonitis, myocarditis, hypophysitis, myositis, inflammatory arthritis, and vasculitis. Because the NIS lacks medication and laboratory data, irAEs were analyzed as ICD-defined inpatient phenotypes rather than adjudicated toxicities. Outcomes included in-hospital mortality, mechanical ventilation as an ICU-level care proxy, length of stay (LOS), and hospital cost derived using cost-to-charge ratios. National estimates incorporated NIS discharge weights, hospital clustering, and stratification. Multivariable survey-weighted logistic regression adjusted for demographics, payer, hospital teaching status, and region. Results: Across 2018–2022, an estimated 4.81 million principal malignancy hospitalizations occurred nationally. The irAE phenotype increased from 3.45% in 2018 to 4.19% in 2022, representing approximately 187,020 weighted irAE-associated admissions. The most prevalent irAE categories were colitis (1.74%) and inflammatory arthritis (1.48%). Compared with non-irAE admissions, irAE phenotype hospitalizations had higher in-hospital mortality (5.12% vs 4.35%), greater mechanical ventilation use (2.86% vs 2.58%), longer LOS (10.7 vs 6.7 days), and higher mean costs ($46.9k vs $29.6k; +$17.3k). After adjustment, the irAE phenotype remained associated with increased mortality (adjusted OR 1.06, 95% CI 1.01–1.12). Resource differentials were greatest in urban teaching hospitals and highest-cost regions, where mean costs reached $67.7k among irAE phenotype admissions. Conclusions: An ICD-10–defined irAE phenotype represents an increasingly prevalent and resource-intensive inpatient comorbidity during cancer hospitalizations, associated with higher mortality, ICU-level care, prolonged LOS, and increased costs. Administrative surveillance of irAE phenotypes may support earlier identification of high-resource admissions and inform inpatient resource planning, particularly in teaching centers and high-cost regions.