CXCL13 and myeloid sarcoma: In contrast to measuring only CSF CXCL13, quantifying intrathecal synthesis of CXCL13 (ICXCL13), as in the use of the IgG index in MS patients, corrects for differences in both serum CXCL13 and blood–CSF barrier integrity between MS patients [36], which can influence the passive transfer of serum CXCL13 into the CSF.