SFTPD and rheumatoid arthritis: The discriminatory power of KL-6 and monomeric periostin to differentiate RA-ILD patients from healthy controls was high (AUC, 0.972 and 0.994) and that of LDH, SP-D, and total periostin was moderate (AUC, 0.886, 0.794, and 0.876, respectively).