Characteristics and Outcomes of Adults Hospitalized With Childhood-Onset Complex Chronic Conditions.

Importance: Individuals with childhood-onset complex chronic conditions (4Cs) are increasingly surviving to adulthood, but the impacts of these conditions on the adult health care system is largely unknown. Objective: To identify and characterize young adults with 4Cs in hospitals and compare their outcomes with other hospitalized young adults. Design, Setting, and Participants: This retrospective cohort study included young adults aged 18 to 39 years who were discharged from 29 hospitals in Ontario, Canada, from January 1 to December 31, 2018. Analyses were performed from November 2024 to April 2025. Exposure: Childhood-onset complex chronic conditions, identified using an adaptation of a widely used pediatric algorithm. Main Outcomes and Measures: Primary outcomes included hospital length of stay, in-hospital mortality, and intensive care unit (ICU) admissions. Secondary outcomes were total cost, number of medications, number of advanced imaging scans, and 30-day readmission rates. Regression weighted by propensity score-based overlap weights was used to assess the association of 4Cs with outcomes at the patient level. Results: Of 19 915 hospitalizations experienced by 15 072 patients (49.6% male; median age, 30 years; interquartile range, 24-35 years), 1329 (6.7%) were associated with 4Cs in 814 individuals (5.4%). The most prevalent pediatric conditions were hereditary anemias (212 of 814 individuals [26.0%]), cystic fibrosis (138 of 814 [17.0%]), and cerebral palsy (96 of 814 [11.8%]). Hospitalizations of patients with 4Cs accounted for 13 606 of 126 910 young adult hospital bed-days (10.7%). Patients hospitalized with 4Cs had lower Charlson Comorbidity Index scores (eg, ≥3: 7 of 757 [0.9%] vs 381 of 13 547 [2.8%]) and were more likely to have longer hospital stays (relative ratio [RR], 1.62; 95% CI, 1.48-1.77), higher total costs (RR, 1.65; 95% CI, 1.05-2.59), more prescribed medications (RR, 1.26; 95% CI, 1.19-1.34), fewer advanced imaging scans (RR, 0.85; 95% CI, 0.77-0.93), and higher 30-day readmission rates (RR, 1.59; 95% CI, 1.28-1.98) compared with those without 4Cs. No significant differences were observed among those with vs without 4Cs for ICU admissions (RR, 1.05; 95% CI, 0.91-1.20) or in-hospital mortality (RR, 1.43; 95% CI, 0.87-2.33). Conclusions and Relevance: In this cohort study, young adults with 4Cs had longer acute care medical hospitalizations, accounting for 10.7% of all young adult hospital bed-days, and more rehospitalizations than other young adults. The results suggest that this population should be prioritized for targeted measures to reduce inpatient stays and improve care. The Charlson Comorbidity Index may be of limited utility for capturing comorbidities in this population.