CSF Immunoglobulin G (IgG) Index

The cerebrospinal fluid (CSF) immunoglobulin G (IgG) index is a laboratory test used to assess the levels of IgG antibodies in the CSF, which surrounds the brain and spinal cord. The test helps diagnose and monitor various neurological conditions.

What is Immunoglobulin G (IgG)?

Immunoglobulin G (IgG) is a type of antibody that plays a crucial role in the immune system. It is the most common type of antibody found in blood circulation and helps protect against bacterial and viral infections.

What is the CSF IgG Index?

The CSF IgG index is a calculated value that compares the levels of IgG in the CSF to the levels of IgG in the blood. The index is calculated by dividing the ratio of IgG to albumin in the CSF by the ratio of IgG to albumin in the blood.

Interpretation of Results

The results of the CSF IgG index are interpreted as follows:

• A normal CSF IgG index is typically less than 0.70.
• An elevated CSF IgG index (greater than 0.70) may indicate intrathecal production of IgG, which can occur in various neurological conditions, such as multiple sclerosis, neurosyphilis, and subacute sclerosing panencephalitis.
• A very high CSF IgG index (greater than 1.0) may suggest a condition characterized by increased intrathecal production of IgG, such as multiple sclerosis or chronic inflammatory demyelinating polyneuropathy.

Clinical Applications

The CSF IgG index has several clinical applications:

• Diagnosis and monitoring of multiple sclerosis: An elevated CSF IgG index can support the diagnosis of multiple sclerosis and help monitor disease activity.
• Diagnosis of neurosyphilis: An elevated CSF IgG index can indicate neurosyphilis, a condition caused by the spread of syphilis to the central nervous system.
• Diagnosis of subacute sclerosing panencephalitis: An elevated CSF IgG index can support the diagnosis of this rare and chronic condition caused by the measles virus.

Limitations and Considerations

The CSF IgG index has some limitations and considerations:

• The test is not specific for any particular disease, and an elevated index can be seen in various conditions.
• A normal CSF IgG index does not rule out neurological disease, as some conditions may not produce a significant increase in intrathecal IgG production.
• The test should be interpreted in conjunction with other laboratory results, clinical findings, and imaging studies to establish a diagnosis.