CXCL13 and rheumatoid arthritis: Further, CXCL13 levels in the serum and joints of RA patients have been found to be an established biomarker in RA14; therefore, RA would be expected to have increased CXCL13 and the lack of a difference between iMCD and RA does not contradict a potential role for CXCL13 in iMCD.