CXCL13 and myeloid sarcoma: Intrathecal production of the protein is measured by calculating the CXCL13 index [(CSF concentrationCXCL13/Serum concentrationCXCL13)/(CSF concentrationalbumin/serum concentrationalbumin)]; neurologists are comfortable with this type of index which is also used for calculation of the IgG index, commonly used in the analysis of CSF in MS.