Test Details: Cerebrospinal fluid (CSF) is a clear, watery liquid that flows around the brain and spinal cord, surrounding and protecting them. CSF testing is performed to evaluate the level or concentration of different substances and cells in CSF in order to diagnose conditions affecting the brain and spinal cord. IgG stands for immunoglobulin G, a type of antibody. Antibodies are proteins made by the immune system to fight viruses, bacteria and other foreign substances. A CSF IgG index measures the levels of IgG in the cerebrospinal fluid. High levels of IgG may indicate an infection, inflammatory or autoimmune disorder.