DPP4 and rheumatoid arthritis: When correlations are analyzed separately for different patient subgroups, we found a trend for a positive correlation between Anti-CD26 IgG levels and DAS28 in RA patients (Spearman R = 0.316, p = 0.073) (Fig. 3), and a significant positive correlation with NAD28 in the RIP group (Spearman R = 0.782, p = 0.038).