MPO and rheumatoid arthritis: MPO activity was significantly higher (36.1-fold increase) in RA-SF (median 32.5 ng/mL, 95% CI 27.1-37.25 ng/mL) than in RA plasma (0.9 ng/mL, 95% CI 0.6-1.6 ng/mL), while MPO activity was hardly detectable in HC plasma samples (Figure 4C).