CD81 and rheumatoid arthritis: Similarly, CD81 levels were significantly higher in RA patients (15.59 ± 2.722 × 1010 particles/mL, p = 0.043) and in AS patients (17.96 ± 2.107 × 1010 particles/mL, p = 0.030) than in OA patients (7.304 ± 2.324 × 1010 particles/mL) (Figure 1D).