Detection and enrichment of autoantibodies in rheumatoid arthritis (RA) synovial extracts. Tissues were obtained by arthroplasty or arthroscopic shaver biopsy, and serum and synovial extracts were analyzed by enzyme-linked immunosorbent assay for antibodies of interest and albumin. (a) Individual levels of RF-IgM, anti-CCP IgG, and anti-tetanus IgG in extracts from osteoarthritis (OA) (n = 12, autoantibodies; n = 11, anti-tetanus) and RA (n = 21, autoantibodies; n = 14, anti-tetanus) synovial tissue, normalized to total protein concentration. Limits of detection are 4 (RF-IgM), 10 (anti-CCP IgG), and 0.3 (anti-tetanus IgG). Serum-normalized levels of (b) RF-IgM and anti-CCP IgG in RA synovial extracts (n = 11) and (c) anti-tetanus IgG in OA (n = 6) and RA (n = 9) synovial extracts. In the box (interquartile range, IQR) and whisker (maximum and minimum) plots, the horizontal line inside the box denotes median and the unfilled circles denote outliers outside IQR ± 1.5 × IQR. The asterisk denotes P = 0.019 by Wilcoxon sign rank test to 1 (no enrichment) for anti-CCP IgG (a), and the indicated P value was determined by Wilcoxon rank sum test between OA and RA for anti-tetanus IgG (b). The value for RF-IgM was not significantly above 1 (P = 0.32). anti-CCP, anti-cyclic citrullinated peptide; RF-IgM, rheumatoid factor of the IgM subtype; SSI, synovial/serum index.