CNS / Demyelinating Antibodies

Beyond the autoimmune encephalitis antibodies (covered separately), several antibodies target other components of the central nervous system — astrocyte water channels (AQP4), oligodendrocyte surface protein (MOG), astrocyte intermediate filaments (GFAP), thyroid antigens (Hashimoto / SREAT), and others. This page is the lab-side reference: what to order, what assay, what sample, titer interpretation, and how the result changes treatment. Clinical disease detail (NMOSD attack management, MOGAD relapse pathway, MS DMT escalation) is left for the Neuroimmunology clinical chapter.

🔹 Bottom Line: CNS / Demyelinating Antibodies

  • AQP4-IgG (NMOSD): serum, cell-based assay gold standard. Positive → mandatory chronic immunosuppression with one of four FDA-approved agents (eculizumab, inebilizumab, satralizumab, ravulizumab). MS DMTs (interferon, natalizumab, fingolimod) harm AQP4+ patients — never start before testing.
  • MOG-IgG (MOGAD): serum, live cell-based assay, IgG1-specific. Titer matters: low titer is common in healthy controls; high titer required for diagnostic confidence. Half are monophasic; relapsing disease often treated with monthly IVIG.
  • GFAP-IgG (astrocytopathy): CSF more sensitive than serum. Linear perivascular radial enhancement on MRI is characteristic; steroid-responsive.
  • Anti-thyroid (TPO, Tg): for Hashimoto encephalopathy / SREAT — diagnosis of exclusion; may be euthyroid; steroid-responsive.
  • High-titer GAD65 (>100× threshold): supports neurologic autoimmunity (stiff-person, cerebellar ataxia, limbic encephalitis, refractory epilepsy). Low titers are common and usually not neurologically significant.
  • Anti-gliadin / anti-TG6: serum, for gluten ataxia — requires high titer + clinical pattern.
  • Test BEFORE immunotherapy when possible; serum is preferred for AQP4 and MOG; CSF is preferred for GFAP.

Anti-AQP4 IgG — NMOSD

Sample & Methodology

  • Sample: serum (more sensitive than CSF).
  • Methodology: cell-based assay (CBA) on live cells is the gold standard. Older indirect immunofluorescence (IIF) has lower sensitivity; ELISA can produce false positives at low titers.
  • Timing: test BEFORE starting immunotherapy when possible — steroids/IVIG/PLEX can transiently lower titers.
  • Repeat in 6–12 weeks if initially negative and clinical syndrome is compelling.

Interpretation

  • Positive result + compatible clinical syndrome → NMOSD (2015 IPND criteria).
  • Titer correlates loosely with disease activity but cannot replace clinical assessment.
  • Low-positive results: confirm with a second CBA run; in clinical context, low-titer AQP4 with NMOSD-typical lesions still supports the diagnosis.

Treatment Impact

  • Positive result mandates chronic immunosuppression — relapses cause cumulative disability.
  • Four FDA-approved options: eculizumab (anti-C5, 2019), inebilizumab (anti-CD19, 2020), satralizumab (anti-IL-6R, 2020), ravulizumab (long-acting anti-C5, 2024).
  • MS DMTs are harmful — interferon-β, natalizumab, fingolimod can worsen disease. Test AQP4-IgG BEFORE starting any MS therapy in atypical or severe demyelinating disease.
  • Acute attacks: high-dose IV methylprednisolone × 5 days; plasmapheresis particularly effective for AQP4-NMOSD attacks — start early when steroid response is inadequate.

Anti-MOG IgG — MOGAD

Sample & Methodology

  • Sample: serum (more sensitive than CSF).
  • Methodology: live cell-based assay — the only reliable method. IgG1-specific detection (total IgG assays give false positives).
  • Persistent positivity at high titer correlates with relapsing course; titers may fluctuate over time.

Titer Interpretation — Critical

  • High titer (≥1:160 or higher depending on assay) in a compatible syndrome → MOGAD with high confidence.
  • Low titer is common in healthy controls, MS, and other conditions — diagnostic confidence requires clinical correlation. 2023 international MOGAD criteria require either high titer OR low titer + additional clinical/radiologic supporting features.
  • Repeat after 1 year in low-titer cases — persistent positivity supports MOGAD; seroreversion supports a single demyelinating event.

Treatment Impact

  • Acute attacks: high-dose steroids with slow taper over months (rapid taper risks rebound). MOGAD is often more steroid-responsive than NMOSD.
  • Relapsing MOGAD: no FDA-approved MOGAD-specific therapy. Monthly IVIG is increasingly first-line; rituximab is an alternative. MS-specific therapies (interferon, natalizumab, fingolimod) are NOT effective and may worsen disease.
  • ~50% of MOGAD is monophasic — chronic immunotherapy is selective, not automatic.

Anti-GFAP IgG — GFAP Astrocytopathy

Sample & Methodology

  • Sample: CSF more sensitive than serum — order BOTH if the syndrome is compelling, but CSF is the primary test.
  • Methodology: cell-based assay; IIF supportive.

Interpretation

  • Positive CSF + linear perivascular radial enhancement on MRI = classical GFAP astrocytopathy.
  • Clinical phenotype: meningoencephalitis ± myelitis ± optic neuropathy ± autonomic dysfunction.
  • ~25% paraneoplastic (ovarian teratoma, others) — tumor workup if positive.

Treatment Impact

  • Often dramatically steroid-responsive. Maintenance immunosuppression for relapsing disease.

Anti-Thyroid Antibodies — Hashimoto Encephalopathy / SREAT

Sample & Methodology

  • Sample: serum.
  • Antibodies: anti-TPO (most common) and/or anti-thyroglobulin (Tg).
  • Thyroid function may be normal — antibody positivity reflects autoimmune predisposition, not necessarily thyroid dysfunction.

Interpretation

  • Anti-TPO and/or anti-Tg positivity in a patient with subacute encephalopathy + myoclonus + tremor + seizures + psychiatric features + elevated CSF protein + diffuse EEG slowing.
  • Diagnosis of exclusion: rule out infectious causes, autoimmune encephalitis with cell-surface antibodies, paraneoplastic syndromes first.
  • “SREAT” (Steroid-Responsive Encephalopathy Associated with Autoimmune Thyroiditis) is the more biochemically honest label.

Treatment Impact

  • High-dose steroids are diagnostic AND therapeutic (clinical improvement supports the label).
  • IVIG or plasmapheresis for steroid-refractory cases.
  • Maintenance immunosuppression for relapsing disease.

Anti-GAD65 Antibodies (Neurologic Use)

Sample & Methodology

  • Sample: serum (CSF can be added — intrathecal synthesis adds confidence in neurologic autoimmunity).
  • Methodology: radioimmunoassay or ELISA.

Titer Interpretation — Critical

  • GAD65 is intracellular — not a directly pathogenic antibody.
  • Low titers (<20 nmol/L or similar threshold) are very common in type 1 diabetes, autoimmune thyroid disease, and healthy controls. NOT clinically significant for neurologic disease.
  • Very high titers (typically >100× the threshold) are required to attribute a neurologic syndrome to GAD65 autoimmunity.

Neurologic Syndromes (High Titer)

  • Stiff-person syndrome (most classic).
  • Cerebellar ataxia (subacute).
  • Limbic encephalitis.
  • Refractory epilepsy (especially temporal lobe).

Treatment Impact

  • Response to immunotherapy is generally poor compared to cell-surface antibody syndromes.
  • Treatment for stiff-person syndrome: high-dose IVIG, plasmapheresis, rituximab; symptomatic with benzodiazepines / baclofen.
  • Refractory epilepsy: immunotherapy trial is reasonable but disappointing in most cases.

Anti-Gliadin / Anti-TG6 (Gluten Ataxia & Neuropathy)

Sample & Methodology

  • Sample: serum.
  • Antibodies: anti-gliadin (older marker), anti-transglutaminase 6 (TG6) is more specific for neurologic gluten sensitivity, anti-transglutaminase 2 (TG2 / tissue TG) for celiac.
  • Pair with HLA-DQ2 / DQ8 typing and duodenal biopsy for celiac diagnosis when celiac is suspected.

Interpretation

  • Positive serology + clinical pattern (idiopathic ataxia or sensory neuropathy in middle-aged adults) supports gluten-related neurologic disease.
  • Anti-TG6 has the strongest specificity for neurologic gluten sensitivity (vs. anti-gliadin which is less specific).

Treatment Impact

  • Strict gluten-free diet is the cornerstone. Antibody titers should fall with adherence.
  • Improvement in ataxia is variable; earlier intervention = better outcome.

Anti-Aquaporin-1 (AQP1) — Emerging

  • Rare; reported in some NMOSD-like presentations that are AQP4-negative.
  • Specialized testing — order only if AQP4 and MOG are negative and the syndrome is NMOSD-like.

Kappa Free Light Chains, OCBs & IgG Index — CSF Markers of Intrathecal Synthesis

Three CSF tests demonstrate intrathecal immunoglobulin production in MS: oligoclonal bands (OCBs), IgG index, and the more recent kappa free light chain (κFLC) index. The 2024 revised McDonald criteria formally accept κFLC index as an alternative to OCBs for demonstrating intrathecal synthesis — the two are now diagnostically interchangeable.

Kappa Free Light Chain (κFLC) Index — The 2024 Update

  • Sample: paired CSF + serum (the index is a ratio).
  • Methodology: validated nephelometric or turbidimetric assay. Fully automated; absolute concentration available in ~30 minutes vs. 4–5 hours for OCB isoelectric focusing.
  • κFLC index calculation: (CSF κFLC / serum κFLC) × (serum albumin / CSF albumin).
  • Cutoff: κFLC index ≥ 6.1 performs best for MS in the 2024 criteria. Individual labs should locally validate; some publish slightly different cutoffs (e.g., 5.9, 6.6).
  • Performance vs OCBs (meta-analysis, 32 studies): diagnostic accuracy is statistically equivalent at the 95% confidence level. κFLC sensitivity ~88% (range 52–100%), specificity ~89% (range 69–100%); OCB sensitivity ~85% (37–100%), specificity ~92% (74–100%). ~90% concordance between the two methods.
  • Applies to both pediatric & adult MS, relapsing-remitting & progressive-onset disease.

Oligoclonal Bands (OCBs)

  • Sample: paired CSF + serum (CSF-restricted bands are what counts).
  • Methodology: isoelectric focusing followed by immunoblot or immunofixation for IgG (gold standard).
  • Definition of positive: ≥ 2 unique CSF bands not present in matched serum.
  • Performance: positive in >90% of MS; usually NEGATIVE in NMOSD (~80% negative) and MOGAD (~70% negative) — OCBs are a strong feature distinguishing MS from the AQP4/MOG demyelinating diseases.
  • Caveats: turnaround 4–5 h; technically more demanding; inter-laboratory variability greater than for κFLC.

IgG Index

  • Calculation: (CSF IgG / serum IgG) × (serum albumin / CSF albumin).
  • Cutoff: > 0.7 supports intrathecal IgG synthesis.
  • Role: supportive only; less sensitive than OCBs or κFLC. Often part of routine CSF panels regardless of MS workup.

🔹 Clinical Relevance: κFLC vs OCBs — Which to Order, and What If They Disagree

The 2024 McDonald criteria treat κFLC index and OCBs as interchangeable for demonstrating intrathecal IgG synthesis. Either can satisfy the CSF biomarker criterion. ~90% of cases agree; the question is what to do with the other 10%.

Practical strategy:

  • Start with κFLC index if your lab offers it (Mayo’s MS cascade does — MSP3 reflex panel) — faster, cheaper, automated, less inter-lab variability.
  • If κFLC index ≥ 6.1 in a compatible clinical syndrome → criterion satisfied; OCBs are NOT also required.
  • If κFLC index is negative but MS is still strongly suspected → reflex to OCBs (or send both up front). κFLC can miss some MS cases that OCBs would catch.
  • If κFLC index is positive but OCBs are negative → the κFLC result still satisfies the McDonald criterion. κFLC can detect intrathecal inflammation in OCB-negative patients (earlier and more sensitively). Document the discordance; clinical syndrome + κFLC positivity is sufficient.
  • If OCBs are positive but κFLC is negative → the OCB result satisfies the criterion. Again, interchangeable.
  • For atypical demyelinating presentations (severe optic neuritis, LETM, area postrema syndrome) — order AQP4-IgG and MOG-IgG before starting MS-specific therapy. OCB positivity helps confirm MS over NMOSD/MOGAD (which are typically OCB-negative).

NfL is NOT a diagnostic marker — the 2024 criteria explicitly recommend against using neurofilament light chain for MS diagnosis (sensitivity only 4–31%). NfL is a prognostic / disease-activity marker, not a diagnostic one.

Mayo MS Testing Cascade

  • MSP3 — Multiple Sclerosis Cascade, Serum and Spinal Fluid. Starts with κFLC, IgG index, and supporting markers; reflexes to OCB testing when indicated.
  • KCSFP — Immunoglobulin Kappa Free Light Chain, Spinal Fluid panel (CSF + serum, with index calculation).
  • Stand-alone OCB panel available when needed for confirmation in the ~10% of discordant cases.

These are CSF tests primarily supporting MS diagnosis. They are not specific to autoimmunity broadly — for NMOSD / MOGAD / GFAP astrocytopathy, the antibody assays above are the diagnostic tests. OCBs / κFLC help distinguish MS from those antibody-mediated diseases (the latter are usually OCB-negative). See the CSF Test Reference Table for the broader CSF panel context.

Order Algorithm for Suspected CNS Demyelinating / Autoimmune Disease

  1. Severe optic neuritis (especially bilateral), LETM (≥3 vertebral segments), area postrema syndrome, or ADEM → AQP4-IgG + MOG-IgG (serum, cell-based assay) BEFORE starting MS therapy.
  2. Atypical “MS” (severe optic neuritis, LETM, prominent brainstem features) → AQP4-IgG + MOG-IgG.
  3. Meningoencephalitis + linear perivascular radial enhancement → CSF GFAP-IgG.
  4. Subacute encephalopathy + myoclonus + EEG slowing + cell-surface antibody negative → anti-TPO ± anti-Tg (SREAT).
  5. Stiff-person syndrome or unexplained subacute cerebellar ataxia → GAD65 (interpret titer carefully).
  6. Idiopathic ataxia or sensory neuropathy in middle-aged adults → anti-TG6 + anti-gliadin + celiac panel.

🔹 Clinical Relevance: Test BEFORE MS Therapy in Any Severe Demyelinating Event

Before AQP4-IgG was discovered (2004), severe demyelinating disease was treated as severe MS — and many patients were started on interferon, natalizumab, or fingolimod, which we now know can worsen NMOSD. Four FDA-approved NMOSD therapies (eculizumab, inebilizumab, satralizumab, ravulizumab) have followed since 2019. The MOG-IgG story is now repeating — 2023 international MOGAD criteria establish it as a third distinct entity with its own treatment pathway. Any severe demyelinating event deserves AQP4 + MOG testing BEFORE starting MS therapy.

Pitfalls and Pearls

  • AQP4-IgG: serum > CSF; live cell-based assay; test before immunotherapy. Positive mandates chronic immunosuppression. MS DMTs are harmful.
  • MOG-IgG: serum; live CBA with IgG1 specificity; titer matters — low titer is common in healthy controls. 2023 criteria require high titer or low titer + supportive clinical/radiologic features.
  • Plasmapheresis especially effective for AQP4-NMOSD attacks.
  • NMOSD treatment: eculizumab, inebilizumab, satralizumab, ravulizumab — all FDA-approved.
  • MOGAD treatment: monthly IVIG increasingly first-line for relapsing disease; rituximab is alternative; ~50% are monophasic.
  • GFAP-IgG: CSF preferred; linear perivascular radial enhancement on MRI; steroid-responsive; ~25% paraneoplastic.
  • Anti-thyroid (TPO/Tg): Hashimoto encephalopathy / SREAT; diagnosis of exclusion; may be euthyroid; steroid-responsive.
  • Anti-GAD65: very high titers only for neurologic significance; intracellular antibody; poor immunotherapy response.
  • Anti-TG6 / anti-gliadin: gluten ataxia; strict gluten-free diet is treatment; TG6 more specific than gliadin.
  • OCBs negative in ~80% NMOSD and ~70% MOGAD; positive in >90% MS — helps distinguish.
  • κFLC index ≥ 6.1 is interchangeable with OCBs in the 2024 McDonald criteria; ~90% concordance. Send the other test only in the ~10% of cases where the first is negative and MS is still strongly suspected.
  • κFLC index advantages: ~30-min turnaround (vs 4–5 h for OCBs), automated, lower inter-lab variability — Mayo’s MS cascade (MSP3) starts with κFLC and reflexes to OCBs.
  • κFLC+ / OCB− in compatible syndrome: κFLC alone satisfies the McDonald CSF criterion. Document discordance; no additional confirmation needed.
  • NfL is NOT a diagnostic marker for MS per 2024 criteria — prognostic / activity marker only.
  • Cell-based assay is the modern standard for surface antigens (AQP4, MOG, GFAP); older IIF and ELISA can give false positives or negatives.
  • Send before immunotherapy when possible — steroids/IVIG/PLEX transiently lower titers.
  • Repeat testing for borderline / low-titer surface antibodies 8–12 weeks apart — persistence supports significance.

References

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  11. Montalban X, Lebrun-Frenay C, Oh J, et al. 2024 revisions to the McDonald criteria for the diagnosis of multiple sclerosis. Lancet Neurol. 2024 — κFLC index accepted as alternative to OCBs for intrathecal synthesis.
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  13. Mayo Clinic Laboratories. MS testing cascade — MSP3 (Multiple Sclerosis Cascade, Serum + Spinal Fluid), KCSFP (κFLC, Spinal Fluid panel). news.mayocliniclabs.com/neurology/demyelinating-disease/multiple-sclerosis/.