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Introduction Pulmonary function impairment and oxygen therapy prescription are used as markers of disease severity in progressive fibrotic interstitial lung disease (PF-ILD), but this relationship has not been studied comprehensively.We aimed to explore the relationship between gas transfer parameters and oxygen therapy initiation in patients with idiopathic pulmonary fibrosis (IPF) and other progressive pulmonary fibrosis (PPF).Methods Single-centre, retrospective analysis of patients with a clinically diagnosed PF-ILD who have had pulmonary function investigations and oxygen assessment within 4 months of each other. Independent sample t-tests were used to compare differences in TLCO at initiation of ambulatory (AOT) and long-term (LTOT) oxygen therapy, as well as between different ILD cohorts at point of oxygen initiation. We calculated the sensitivity and specificity of multiple TLco thresholds to discriminate the need for LTOT.Results 119 patients (88 male, 52 IPF) were included. Mean transfer factor of the lung for carbon monoxide (TLco) of patients with PF-ILD at the closest timepoint to AOT and LTOT initiation was 41.2%(9.0, mean±SD) and 34.9%(6.5) respectively and 52.3%(8.6) in the no oxygen therapy cohort (table 1). There was a significant difference in mean TLco at AOT initiation and LTOT initiation in the IPF cohort (p<0.05). There were no significant differences in mean TLCO between IPF and PPF patients at the point of oxygen initiation. A 40% of predicted TLco threshold had a sensitivity of 92% and specificity of 89% for LTOT eligibility. Of interest, in the IPF cohort, 42% of patients had a forced vital capacity (FVC) >80% at initiation of AOT, but 0% of IPF patients that went on to LTOT had an FVC of >80%.Conclusions Due to finding a highly sensitive and specific TLco threshold, this study has implications for the timely identification of PF-ILD patients most appropriate for oxygen assessment to ensure earlier treatment initiation.Abstract P12 Table 1Mean TLCO at closest time point to oxygen therapy initiation or assessmentNTLCO (%pred)PF-ILD No O24752.3(8.6)PF-ILD AOT7041.2(9.0)PF-ILD LTOT1834.9(6.5)IPF No O21752.6(8.0)IPF AOT3240.4(8.1)*IPF LTOT932.8(2.8)*PPF No O23052.1(8.9)PPF AOT3741.9(9.7)PPF LTOT936.9(8.2)Mean (±SD), * denotes a significant difference between cohorts