Test Details: A CSF IgG index measures the levels of IgG in the cerebrospinal fluid. High levels of IgG can indicate an autoimmune disorder. An autoimmune disorder causes the immune system to attack healthy cells, tissues, and/or organs by mistake. If CSF IgG index shows higher than normal levels, it may indicate multiple sclerosis, other autoimmune disease, such as lupus or rheumatoid arthritis.