PADI4 and rheumatoid arthritis: The levels of SF-citrullinated PAD4 in the low-titer group, including anti-CCP negative patients with RA (n = 3), and in the high-titer group were significantly higher than those in patients with OA (0.164 ± 0.091, 0.174 ± 0.055, and 0.046 ± 0.035 ng/mL, respectively; P < 0.05; Fig. 4c).