Prognostic significance of the ROX index in patients with acute exacerbation of interstitial lung disease treated with high-flow nasal cannula oxygen therapy: a retrospective cohort study

BACKGROUND: The respiratory rate-oxygenation (ROX) index (calculated as [SpO₂/FiO₂ ratio] / respiratory rate) is reported to be associated with outcomes in patients with acute hypoxemic respiratory failure treated with a high-flow nasal cannula (HFNC). However, its prognostic utility in acute exacerbation of interstitial lung disease (AE-ILD) remains unclear. METHODS: We conducted a single-center retrospective cohort study that included patients with AE-ILD who received HFNC between April 2016 and June 2023. The ROX index at HFNC initiation (0 h) and 12 h was recorded. The primary outcome was 30-day mortality. Logistic regression analysis was also performed. RESULTS: In total, 92 patients were included in this study; their median age was 80 years, and 73% were male. Univariate analysis revealed that 30-day mortality was associated with the ROX index at 0 h (odds ratio [OR], 0.80; 95% confidence interval [CI]: 0.63-1.00; p = 0.048) and 12 h (OR, 0.66; 95% CI: 0.50-0.85; p = 0.001). Multivariate analysis revealed that the ROX index at 12 h was independently associated with 30-day mortality (OR, 0.67; 95% CI: 0.51-0.88; p = 0.003). The optimal cutoff value of the ROX index at 12 h for 30-day mortality was 5.82 (area under the curve: 0.76). CONCLUSIONS: The ROX index at 12 h after HFNC was associated with 30-day mortality in patients with AE-ILD and may serve as a simple bedside marker for short-term prognosis.