GDF15 and idiopathic pulmonary fibrosis: Plasma levels of GDF15 were significantly higher in HP patients compared to HC (2131 ± 886 pg/ml versus 1047.8 ± 877 pg/ml; p < 0.0001) although they did not reach the levels observed in IPF patients (2923 ± 790 pg/ml) (Fig. 1).