IgG index

The article was written by MSc Tran Thi Huyen Trang - Laboratory Doctor, Laboratory Department - Vinmec Times City International General Hospital

IgG index is an index calculated by the formula [IgG CSF*Albumin blood]/[IgG blood* Albumin CSF] where CSF stands for cerebrospinal fluid. An increase in the IgG index reflects increased production of IgG in the central nervous system. To calculate the IgG index, 4 parameters must be tested at the same time: IgG blood, IgG CSF, Blood Albumin, Albumin CSF.

The IgG index is an index calculated by the formula [IgG CSF*Albumin blood]/[Blood IgG* Albumin CSF] where CSF stands for cerebrospinal fluid. An increase in the IgG index reflects increased production of IgG in the central nervous system. To calculate the IgG index, 4 parameters must be tested at the same time: IgG blood, IgG CSF, Albumin blood, Albumin CSF.
The index is used to evaluate the pathology of the central nervous system, especially to support the diagnosis of multiple sclerosis (MS). Multiple sclerosis is an autoimmune disorder affecting the central nervous system through the process of demyelination. The disease usually begins at the age of 20-40, worsens if not diagnosed and treated early.

MS is considered the most common non-stroke-related cause of disability in young adults. Using the IgG Index can help doctors diagnose disease at an early stage, and it is also a simple, low-cost and easily accessible tool.
Besides MS, there are other causes of increased CSF IgG (increased IgG index) such as: neurosyphilis, acute polyneuritis, measles encephalitis (Dawson's disease).

IgG index > 0.7 is a predictive marker for the presence of oligoclonal IgG bands in cerebrospinal fluid protein electrophoresis, which means the presence of MS at an early stage. The IgG index is elevated in approximately 80% of patients with multiple sclerosis, and presences of more than or equal to 2 oligoclonal IgG bands in cerebrospinal fluid protein electrophoresis are also present in 80% of MS cases. Therefore, the IgG index combined with CSF protein electrophoresis increases the MS diagnostic sensitivity up to 90%. At the same time, the IgG index can be used as a substitute for CSF protein electrophoresis in facilities where CSF electrophoresis cannot be performed.

Normally, IgG levels remain insignificant in the CSF, mainly CSF IgG derived from peripheral blood after crossing the blood-brain barrier. Therefore, the presence of IgG in the cerebrospinal fluid does not imply central nervous system inflammation. However, in one of the diseases mentioned above, especially MS, IgG is produced in the CNS by B lymphocytes leading to a change in the correlation between CSF and blood IgG.
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