GDF15 and idiopathic pulmonary fibrosis: The percentages of GDF-15- expressing cells in the lung of AE-IPF patients (23.67 ± 1.43%) were more than both HCs (15.34 ± 1.38%) and S-IPF (17.66 ± 1.10%) subjects (p < 0.001 and p < 0.001, respectively) (5 fields each sample).