GDF15 and interstitial lung disease: ELISA-based analysis of plasma samples from n = 81 COVID-19 patients, n = 20 influenza A patients, n = 11 ILD patients, and n = 6 ILD patient with acute exacerbation showed increased levels of the fibrogenesis markers thrombospondin 2 (TSP2), glial derived factor 15 (GDF15), insulin like growth factor binding protein 7 (IGFBP7), cluster of differentiation 163 (CD163), and propeptides of procollagen type III, IV and V (PROC3, PROC4, PROC5) in COVID-19 compared to influenza A and ILD entities.