The burden of Progressive Fibrotic Interstitial lung disease across the UK

While Idiopathic pulmonary fibrosis (IPF) remains the exemplar progressive fibrotic lung disease, there remains a cohort of non-IPF fibrotic lung diseases (fILD) which adopt a similar clinical behaviour to IPF despite therap. This phenotypically related group of conditions, where progression of disease is similar to that seen in IPF, have recently been described as Progressive Fibrotic Interstitial Lung diseases (PF-ILD). Previous estimates suggest that between 18 to 40% of all fILD will develop progressive disease, however, the exact burden remains unknown. This retrospective, observational study therefore aimed to estimate the incidence of PF-ILD across England. All new referrals seen across nine UK centres for their first outpatient clinic appointment between 1st August 2017 and 31st January 2018 were assessed against the diagnostic criteria for PF-ILD laid out in the INBUILD trial. A total of 1749 patients with fILD were assessed. In this cohort of patients at risk of developing PF-ILD the INBUILD criteria were met in 14.5% (253/1749) of all new non-IPF fILD referrals. The average time from referral to specialist centre to diagnosis of progressive phenotype was 311 days. Of the progression events the majority were driven by a measured drop in FVC, with more than half of patients experiencing a drop of 10%. Almost one quarter of patients (24.1%) were diagnosed with progressive disease on the basis of radiological and symptomatic progression alone without a spirometric deterioration. This study represents a fair and balanced approach to assessing the incidence of objectively measurable and treatable PF-ILDs in the UK. A rate of 14.5% of new referrals with non-IPF ILD is less than that reported in previous studies however our methodology is likely to give a more accurate result than estimates based on extrapolation from general disease statistics, from physician-reported estimates prone to significant biases, or insurance claim processes also substantially prone to bias. This information has implication for workforce planning and the funding of anti-fibrotic therapy in the UK and beyond.