IgG index
A calculated ratio that determines whether elevated CSF immunoglobulin G represents local CNS synthesis or systemic leak.
Full Definition
The IgG index is a mathematical calculation that compares the ratio of immunoglobulin G in cerebrospinal fluid to serum with the ratio of albumin in CSF to serum. This index helps distinguish between intrathecal (local central nervous system) immunoglobulin synthesis and passive transfer from blood due to blood-brain barrier dysfunction. An elevated IgG index (>0.7) suggests local antibody production within the CNS, which is characteristic of inflammatory neurological conditions such as multiple sclerosis. The calculation normalizes for blood-brain barrier permeability, making it more specific than absolute CSF IgG levels alone.
Usage
Usage note: Always express as a decimal value; normal range is typically <0.7.
In Context
- "The patient's IgG index of 0.85 indicated intrathecal immunoglobulin synthesis." — CSF analysis report
- "An elevated IgG index supports the diagnosis of central nervous system inflammation." — Diagnostic workup