S100A9 and idiopathic pulmonary fibrosis: The ROC curve of BALF and serum S100A9 levels in the IPF group (Fig. 2), had the area under the curve of 0.67 and 0.44 (both P > .05), respectively indicating that S100A9 expression in BALF or serum could not be used as a reliable marker for the diagnosis of IPF in this study.