Long-term oxygen therapy (LTOT) is universally accepted as standard care for patients with severe resting hypoxaemia, irrespective of underlying diagnosis, with the aim being to improve patient survival. COPD and fibrotic interstitial lung disease (ILD) are the two most common indications for LTOT globally [1][2][3][4]. As therapeutic benefits of LTOT have only been established in patients with COPD [5,6], current prescribing recommendations are based on the study entry criteria used in the pivotal clinical trials of LTOT in COPD [5,6]. Patients with ILD have significantly worse survival after commencing LTOT than patients with COPD [7, 8]. Despite fibrotic ILD and COPD sharing similar clinical presentations, there are marked differences in their pathophysiology. The significance of differing severities of resting hypoxaemia and therapeutic effects of LTOT have not been explored in the ILD population [9]. It is unknown whether the current oxygen prescribing threshold for LTOT established in patients with COPD is appropriate for patients with fibrotic ILD. This study aimed to evaluate the significance of moderate resting hypoxaemia in fibrotic ILD. We hypothesised that moderate resting hypoxaemia would be an independent prognostic factor for survival in ILD and there would be a difference in survival between fibrotic ILD and COPD patients with moderate resting hypoxaemia.Consecutive patients with a multidisciplinary diagnosis of fibrotic ILD who had undergone assessments to determine the need for domiciliary oxygen therapy between January 2011 and December 2018 were identified using the ILD registries at two quaternary hospitals in Melbourne, Australia: Austin and Alfred Health. All patients completed 6-min walk tests (6MWTs) on room air and measurement of arterial blood gases (ABG), if resting oxyhaemoglobin saturation (S pO 2 ) was ⩽94%. Data collected included patient demographics, diagnosis, date and details of initial oxygen assessments (6MWT, ABG), lung function tests within 1 month of initial oxygen assessments (spirometry, diffusing capacity for carbon monoxide (D LCO )), and survival or lung transplantation status. Degree of resting hypoxaemia was classified as 1) moderate: arterial oxygen tension (P aO 2 )56-59 mmHg without hypoxic organ damage or 60-65 mmHg [10]; or 2) severe: P aO 2 ⩽55 mmHg or 56-59 mmHg with hypoxic organ damage [11]. Isolated exertional hypoxaemia was defined as nadir S pO 2 of ⩽88% during 6MWT on room air [12]. A cohort of consecutive patients with COPD and moderate resting hypoxaemia were identified using the Austin Health Oxygen Service database for comparison. This study was approved by the Austin Health Human Research Ethics Committee (LNR/18/Austin/388, LNR/19/Austin/45).Statistical analyses were performed using Stata (v15.1 StataCorp, USA). Descriptive analyses are presented as mean±SD or median (interquartile range). Survival following initial oxygen assessments was determined using the Kaplan-Meier method, with events being death or lung transplantation. Survival time was c...

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