Plasma matrix metalloproteinase (MMP) 7 of > 1.75 ng/mL, surfactant protein D (SP-D) of > 31 ng/mL, and osteopontin of > 6 ng/mL differentiated patients with IPF from patients with other idiopathic interstitial pneumonias, but not from patients with rheumatoid arthritis-associated ILD (RA-ILD).27 Here, SPP1 is linked to rheumatoid arthritis.