External validation of predictive equations for determining the need for in-flight oxygen supplementation in patients with respiratory disease.

Rationale: Patients with chronic respiratory diseases may develop clinically significant hypoxemia during commercial air travel because of reduced cabin pressure. In-flight oxygen prescription is typically guided by hypoxic challenge testing (HCT), a procedure that is time-consuming, costly, and not universally available. Several predictive equations based on sea-level physiological measurements have been proposed as alternatives. We aimed to externally validate six published predictive equations by comparing their performance with HCT in a large, heterogeneous cohort of patients with chronic respiratory diseases. Methods: We retrospectively analyzed all patients who underwent HCT at our center between 2016 and 2026. Sea-level arterial blood gases, pulmonary function tests, and relevant clinical data were collected. Six published predictive equations (E1-E6) were used to estimate in-flight PaO Results: A total of 177 patients were included: 39 (22%) with COPD, 64 (36%) with ILD, 34 (19%) with cystic fibrosis, and 40 (23%) with other chronic respiratory diseases. Significant correlations between predicted in-flight PaO Conclusions: In this large external validation study, currently available predictive equations demonstrated insufficient diagnostic performance to safely replace HCT for determining the need for in-flight oxygen supplementation. Although several equations showed moderate to strong agreement with measured HCT PaO