Fibroblast activation protein and disease severity, progression, and survival in idiopathic pulmonary fibrosis.

Idiopathic pulmonary fibrosis (IPF) is characterized by progressive fibrosis in the lungs. Activated fibroblasts play a central role in fibrogenesis and express fibroblast activation protein α. A truncated, soluble form (sFAP) can be measured in blood and is a potential novel biomarker of disease activity. The aim was to study the association between sFAP and clinical, radiological, and histopathological measures of disease severity, progression, and survival in a prospective, multicentre, real-world cohort of patients with IPF. Patients with IPF were recruited from the tertiary interstitial lung disease centres in Denmark and followed for up to 3 years. Baseline serum levels of sFAP were measured by ELISA in patients with IPF and compared to healthy controls. Pulmonary function tests, 6-minute walk test and quality of life measures were performed at baseline and during follow-up. The study included 149 patients with IPF. Median sFAP in IPF was 49.6 ng/mL (IQR: 43.1-61.6 ng/mL) and in healthy controls 73.8 ng/mL (IQR: 62.1-92.0 ng/mL). Continuous sFAP was not associated with disease severity, progression or survival (p > 0.05). After dichotomization of sFAP below or above mean sFAP + 2 SD for healthy controls, higher levels of sFAP were associated with lower FVC % predicted during follow-up (p < 0.01). Higher than normal serum levels of sFAP were associated with longitudinal changes in FVC % predicted, but sFAP did not show clear associations with other baseline or longitudinal parameters. As such, sFAP has limited use as a biomarker of disease progression or survival in patients with IPF.