PRG4 and rheumatoid arthritis: Notably, compared with control SF, the concentration of lubricin declined by 58.2% in lOA SF [152.3 μg/ml (108.2–183.9 μg/ml), p = 0.005] and by 61.7% in RA SF (139.4 μg/ml (124.6–162.4 μg/ml), p<0.001].