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RATIONALE Chronic fibrosing interstitial lung disease (ILD) with a progressive phenotype is a clinical concept describing the broad group of interstitial lung diseases characterized by progressive pulmonary fibrosis. The prevalence of progressive fibrotic ILDs other than idiopathic pulmonary fibrosis (IPF) is not well understood. OBJECTIVES We utilized a novel algorithm to estimate the prevalence range of disease progression among patients with non-IPF fibrotic ILD in a US claims database. METHODS This was a retrospective study including adults with commercial or Medicare Advantage with Part D (MAPD) insurance using administrative claims data from October 2015 - September 2019. Patients likely to have non-IPF fibrosing ILDs with a progressive phenotype were identified via an algorithm that incorporated ILD-related diagnosis codes (excluding IPF) and claims-based proxies for fibrotic ILD progression, including pulmonary function tests, chest imaging, oral corticosteroid (OCS) medications, immunosuppressive medications, lung transplant, oxygen therapy, palliative care, and respiratory hospitalization. The prevalence range of non-IPF fibrotic ILD with progressive disease behavior was calculated using strict and lenient case definitions to account for potential imprecision in the progression proxies. RESULTS Of nearly 9 million study-eligible patients, 17,136 were identified with non-IPF fibrosing ILD. The prevalence of disease progression per 10,000 (95% CI) ranged from 12.14 (11.74-12.54) to 29.05 (28.43-29.67) over a mean observation time of 1.44 years for MAPD enrollees (n = 14,686), and from 0.89 (0.81-0.97) to 2.36 (2.24-2.48) over a mean observation time of 1.29 years for commercial enrollees (n = 2,450). Prevalence estimates increased with age for both insurance types. Among patients with progression, 4,097 met ≥ 2 progression proxies not considering OCS (strict case definition) and 9,946 met ≥ 1 progression proxy (lenient case definition). The mean (SD) number of proxies met was 2.1 (1.3) and the most common individual proxies met (alone or in combination with other proxies) were OCS use (48.9%), respiratory hospitalization (44.2%), and oxygen therapy (44.1%). CONCLUSIONS This is among the first claims-based estimates of the prevalence of non-IPF chronic fibrosing ILD with a progressive phenotype. Our analysis indicates that this phenotype is rare in the overall population but increases substantially with increasing age.