Correlation between synovial/serum index (SSI) and mRNA for total IgM and IgG. Correlation plots for SSI versus GAPDH-normalized mRNA levels for total IgM (a) and total IgG (b) in rheumatoid arthritis (RA) and osteoarthritis (OA) synovia. Data are from Figure 2. Correlation coefficients (R) and P values were determined by Spearman rank correlation. GAPDH, glyceraldehyde-3-phosphate dehydrogenase; REU, relative expression units.