Scatterplots illustrate strong correlation of serum CXCL13 with seropositivity and immunoglobulin M rheumatoid factor in the Dartmouth RA Cohort. (A) Log-transformed C-X-C motif chemokine 13 (CXCL13) values are higher in seropositive compared to seronegative rheumatoid arthritis (RA) patients by t-test (seropositive (n = 163), geometric mean (95% CI) = 331.1 pg/ml (250.0 to 430.5); seronegative (n = 30), geometric mean (95% CI) 93.3 pg/ml (71.3 to 123.9); P = 0.0002). (B) The log-transformed CXCL13 values of seropositive patients increase with higher rheumatoid factor (RF) levels when evaluated by Spearman correlation (P < 0.0001). (C) Tertile analysis of seropositive patients comparing the highest CXCL13 tertile (third tertile) to the first and second tertiles by Wilcoxon rank-sum test identifies a strong relationship with immunoglobulin M (IgM) RF (third-tertile IgM RF: mean = 182 ± 59 IU/ml; first- and second-tertile IgM RF: mean = 113 ± 74 IU/ml; P < 0.0001). (D) CXCL13 has a weaker relationship to IgG anticitrullinated peptide/protein antibody (ACPA) (P = 0.03). *P < 0.05. Diagonal lines represent line of best fit.