Platform
Company
Microbiology & Infection
Review status
Currently under review
Pending specialist review and validation.
The CSF IgG Index is a calculated measure that compares immunoglobulin G (IgG) in your cerebrospinal fluid to IgG in your blood, adjusted for albumin. It helps determine whether your immune system is producing antibodies within the central nervous system rather than IgG simply leaking in from the bloodstream.
To perform the test, a clinician collects cerebrospinal fluid with a lumbar puncture and a blood sample, ideally close in time to each other. The index is often ordered along with other CSF studies, such as cell counts, protein, glucose, oligoclonal bands, and targeted tests for infections.
This test helps differentiate a general barrier disturbance from true antibody production inside the brain and spinal cord. It is used when your clinician is evaluating inflammatory or infectious conditions of the nervous system, such as suspected multiple sclerosis, chronic meningitis, Lyme neuroborreliosis, neurosyphilis, or encephalitis. It complements MRI findings and other CSF tests like oligoclonal bands.
Results can guide whether further infectious testing, autoimmune panels, or treatment is needed. The lumbar puncture required for this test carries small risks, including headache and temporary back discomfort, which your care team will discuss with you in advance.
If your index is higher than your lab’s normal limit, it suggests that your immune system is making IgG within the central nervous system. This supports, but does not by itself prove, diagnoses such as multiple sclerosis or certain infections. Your clinician will interpret the result together with your symptoms, examination, MRI findings, and other laboratory studies.
A result within the normal interval does not rule out disease, especially early or localized conditions. If findings are unexpected, your team may evaluate for blood contamination of the spinal fluid, confirm that blood and CSF were collected the same day, repeat testing, or add studies such as oligoclonal bands and pathogen-specific assays. Discuss any new or worsening neurological symptoms promptly so follow-up can be tailored to you.
Reference intervals vary by laboratory, analyzer, methodology, population, and units. The ranges shown here are for education only. Always interpret your results against the reference interval printed on your own lab report.
The index requires both CSF and blood collected close together. If the serum sample is missing or taken far from the lumbar puncture time, the calculation can be unreliable or cannot be performed.
Blood contamination of the CSF can introduce serum IgG and albumin, falsely affecting the index. Laboratories often correct or comment, but significant contamination may necessitate repeat sampling.
Infusions of immunoglobulin or antibody-based drugs can temporarily elevate serum and sometimes CSF IgG, altering the index. Tell your clinician about recent treatments so results are interpreted appropriately.
Medications that dampen the immune system can reduce intrathecal antibody production and may lower the index even in active disease, potentially masking inflammation.
Inflammatory conditions in the brain or spinal cord can increase intrathecal IgG production and raise the index. The result should be considered alongside cultures, PCR panels, serology, and imaging.
Physiologic changes that affect the blood-brain barrier or albumin transport can influence the index. Your clinician will interpret results in clinical context for special populations.
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