Scatterplots demonstrate serum CXCL13 strongly correlates with seropositive rheumatoid arthritis and immunoglobulin M rheumatoid factor levels in the Sherbrooke Early Undifferentiated PolyArthritis Cohort, but has a weaker relationship to anticitrullinated peptide/protein antibody levels. (A) Log-transformed C-X-C motif chemokine 13 (LogCXCL13) is higher in seropositive than in seronegative RA patients as determined by t-test (seropositive: n = 173, geometric mean (95% CI) = 323.6 pg/ml (223.9 to 477.5); seronegative: n = 166, geometric mean (95% CI) = 50.1 pg/ml (35.0 to 78.0); P < 0.0001). (B) As with the Dartmouth RA Cohort, a strong relationship is seen with LogCXCL13 and immunoglobulin M rheumatoid factor (IgM RF) in seropositive samples, whether measured by Spearman correlation (P < 0.0001) (C) or by CXCL13 tertile analysis (third-tertile mean RF = 223 ± 57 IU/ml, first- and second-tertile mean RF = 141 ± 75 IU/ml; P < 0.0001). (D) A significant relationship is found between CXCL13 and anticitrullinated peptide/protein antibody (ACPA) by Spearman correlation (P = 0.006). *P < 0.05. Diagonal lines represent lines of best fit.