How do you calculate IgG index?

The IgG index is calculated to assess the presence of intrathecal antibody production, particularly in neurological disorders. It is determined by measuring the levels of IgG in the cerebrospinal fluid (CSF) and serum. The formula is: IgG index = (CSF IgG level / Serum IgG level) / (CSF albumin level / Serum albumin level). An elevated IgG index suggests local IgG synthesis in the central nervous system.