The IgG Index test evaluates the production of immunoglobulin G (IgG) within the central nervous system. This index is calculated by comparing IgG levels in the CSF to those in the serum, helping detect conditions like multiple sclerosis or other inflammatory diseases affecting the brain and spinal cord. A higher IgG index suggests abnormal IgG synthesis within the CNS, potentially indicating neurological inflammation. This test aids in differential diagnosis and is typically ordered alongside other CSF studies for comprehensive assessment.