In human patients, serum PRG4 was unchanged in cases of rheumatoid arthritis (35) (0.53 to 1.41 μg/mL, measured by ELISA) and advanced joint disease (39) (0.25 to 1 μg/mL, measured by commercial ELISA kit), whereas plasma PRG4 was associated with joint space narrowing, after adjusting for age and sex (40). Here, PRG4 is linked to arthropathy.