A, CD13 was measured by enzyme-linked immunosorbent assay (ELISA) and enzymatic activity in RA (n=39) and OA (n=59) synovial fluids. B, CD13 was measured by ELISA in sera from healthy controls (n=23), sera from RA patients (n=33) and in synovial fluids from RA patients (n=39, not matched to serum). * p≤0.05, ****p≤0.00001, by One-way ANOVA