CSF IgG Index Calculator
CSF IgG Index Calculator
Calculate the CSF IgG index to detect intrathecal immunoglobulin synthesis, correcting for blood-brain barrier permeability.
CSF IgG Index
(CSF/serum IgG) ÷ (CSF/serum albumin)CSF IgG 5.5 mg/dL, serum IgG 1100 mg/dL, CSF albumin 24 mg/dL, serum albumin 4200 mg/dL
Formula
- CSF/serum IgG ratio
- how much IgG is in CSF relative to blood
- CSF/serum albumin ratio
- the barrier-permeability correction — albumin is made only in the liver, so any albumin in CSF crossed the barrier
- units
- CSF and serum values for each analyte must be in the SAME units before dividing
Worked example
CSF IgG 5.5 mg/dL, serum IgG 1100 mg/dL, CSF albumin 24 mg/dL, serum albumin 4200 mg/dL
CSF/serum IgG ratio = 5.5 ÷ 1100 = 0.0050
CSF/serum albumin ratio = 24 ÷ 4200 = 0.00571
0.0050 ÷ 0.00571 = 0.88 — raised, consistent with intrathecal IgG synthesis
IgG index versus oligoclonal bands
| Test | Sensitivity for MS | Nature of result |
|---|---|---|
| IgG index > 0.7 | ≈ 70% | Quantitative, cheap, widely available |
| Oligoclonal bands (isoelectric focusing) | ≈ 95% | Qualitative, the reference standard |
What the index corrects for
The CSF IgG index distinguishes IgG that has been synthesised inside the central nervous system from IgG that has simply leaked across a damaged blood-brain barrier. Dividing the CSF-to-serum IgG ratio by the CSF-to-serum albumin ratio makes this possible, because albumin is produced only in the liver — any albumin found in CSF must have crossed the barrier, so the albumin ratio measures barrier permeability on its own, independent of any immune process. Dividing the IgG ratio by it corrects for that permeability and isolates the IgG that was actually made within the CNS.
An index above 0.7 indicates intrathecal IgG synthesis and is found in roughly 70% of patients with multiple sclerosis. It is, however, less sensitive than isoelectric focusing for oligoclonal bands, the reference method, which detects around 95% — the index is best treated as a quantitative complement to oligoclonal banding rather than a replacement for it.
It is also not specific for multiple sclerosis. Neurosyphilis, subacute sclerosing panencephalitis and CNS lupus all raise it, so a positive result needs interpreting alongside the clinical picture and other CSF findings. And before calculating, ensure both albumin values and both IgG values are expressed in the same units — mixing mg/dL with g/L is a recurring source of a badly wrong index.
Frequently asked questions
What does the CSF IgG index measure?
Whether IgG in the CSF was synthesised within the central nervous system rather than having leaked in from blood. It corrects the CSF-to-serum IgG ratio for blood-brain barrier permeability using the albumin ratio.
What IgG index suggests multiple sclerosis?
Above 0.7, found in around 70% of MS patients. It is less sensitive than oligoclonal bands by isoelectric focusing, which detect roughly 95% and remain the reference test.
Is a raised IgG index specific for MS?
No. Neurosyphilis, subacute sclerosing panencephalitis and CNS lupus can all raise it, so the result needs interpreting alongside the clinical picture rather than read alone.
Do the units matter for this calculation?
Yes, critically. CSF and serum IgG must be in the same units as each other, and CSF and serum albumin must be in the same units as each other, or the ratios — and the index — are meaningless.
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References
- Link H, Tibbling G. Principles of albumin and IgG analyses in neurological disorders. Scand J Clin Lab Invest. 1977;37(5):385–90.
- Freedman MS et al. Recommended standard of cerebrospinal fluid analysis in the diagnosis of multiple sclerosis. Arch Neurol. 2005;62(6):865–70.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.