When should serum anti‑myelin oligodendrocyte glycoprotein (anti‑MOG) IgG testing be performed, how are the results interpreted, and what are the recommended treatment options for MOG‑associated disorder?

Serum Anti-MOG Testing: When, How, and What to Do

Test serum anti-MOG IgG using cell-based assays in patients presenting with optic neuritis (especially recurrent or bilateral), longitudinally extensive transverse myelitis, acute disseminated encephalomyelitis (ADEM), or brainstem encephalitis—but avoid indiscriminate screening as this reduces positive predictive value and leads to overdiagnosis. 1

When to Order Anti-MOG IgG Testing

High-Yield Clinical Scenarios (Test These)

Based on international consensus guidelines, order testing when patients present with 1:

• Optic neuritis with:

  • Recurrent episodes
  • Bilateral simultaneous involvement
  • Prominent optic disc edema
  • Perineural enhancement on MRI 2

• Myelitis presentations:

  • Longitudinally extensive transverse myelitis (LETM)
  • Conus involvement
  • Severe attacks with residual deficits 3

• ADEM-like presentations:

  • Particularly in children
  • Multifocal white matter lesions with encephalopathy 1, 4

• Brainstem or cerebellar syndromes with demyelinating features 1

• Cerebral cortical encephalitis patterns 5

Critical Pitfall: Don't Screen Everyone

Do NOT routinely test patients with typical multiple sclerosis presentations—screening large unselected cohorts for this rare biomarker significantly reduces positive predictive value and leads to false positives and inappropriate treatment 1. The phenotypic overlap between MOG-EM and MS is substantial, but indiscriminate testing causes more harm than good.

How to Test: Technical Requirements

Assay Methodology

• Use cell-based assays (CBA) with full-length human MOG only 1
• Old ELISA methods using denatured MOG peptides are obsolete and should not be used 1
• Test for MOG-IgG specifically—do not test for IgM or IgA as their clinical relevance is unknown 1

Specimen Requirements

• Serum is the specimen of choice (ship at 4°C or on dry ice) 1
• CSF testing is not usually required since MOG-IgG is produced extrathecally, resulting in lower CSF than serum titers 1
• CSF may be helpful only in rare cases with high-titer non-MOG serum antibodies causing background interference 1

Timing Matters Critically

MOG-IgG concentrations fluctuate based on 1, 6:

• Disease activity: Higher during acute attacks than remission
• Treatment status: Lower while on immunosuppression
• Post-plasma exchange: May transiently vanish

If initial test is negative but suspicion remains high:

• Retest during acute attacks
• Retest during treatment-free intervals
• Wait 1-3 months after plasma exchange or IVIG before retesting 1

Critical timing insight: In patients tested within 6 months of clinical onset, approximately 25% can become low-positive by 6 months 6. Delayed testing reduces sensitivity.

Interpreting Results

Positive Results: Apply "Red Flags"

A positive MOG-IgG must be interpreted in clinical context. Challenge the result if these "red flags" are present 1:

• Progressive MS course (primary or secondary progressive)
• Intrathecal oligoclonal bands or MRZ reaction (measles/rubella/zoster antibodies)—these suggest MS, not MOG-EM
• Very high CSF white cell count (>100 cells/μL can occur in MOG-EM but should prompt consideration of CNS infection)
• CSF MOG-IgG titer higher than serum (MOG-IgG is produced extrathecally)
• Double-positive for AQP4-IgG and MOG-IgG (extremely rare; retest both)
• Combined central and peripheral demyelination (MOG is not expressed in peripheral nervous system)
• Clinical/radiological features suggesting alternative diagnoses (infection, vasculitis, lymphoma, etc.) 1

Low-Positive Results Require Extra Scrutiny

Low-positive titers are more common with longer intervals from disease onset to sampling (p < 0.001) 6. The 2023 MOGAD criteria require additional supportive clinical/MRI features when MOG-IgG is low-positive to maintain specificity 5, 7.

If low-positive and "red flags" present:

• Retest using a second, methodologically different cell-based assay 1
• If discrepant, use a third assay
• Seek expert consultation from specialized center 1

Negative Results: When to Retest

Some patients with monophasic disease may lose MOG-IgG over time, particularly children with ADEM 1. However, patients with relapsing disease typically remain positive. Consider retesting 6-12 months after initial attack for prognostic information 1.

Treatment Approach for MOG-Associated Disorder

Acute Attack Management

First-line: Intravenous methylprednisolone (IVMP) for 3-5 days, followed by prolonged oral steroid taper over 3 months 8, 9. MOG-EM carries high risk of flare-ups after abrupt steroid cessation 1.

Second-line (if inadequate response to IVMP):

• Plasma exchange or immunoadsorption—MOG-IgG positive patients may be particularly responsive 1, 8, 9
• Intravenous immunoglobulin (IVIG), especially in children 1, 2, 9

Long-Term Immunotherapy

Reserve maintenance treatment for 2, 8, 9:

• Patients with relapsing disease
• Severe initial presentation with poor recovery
• Persistent high MOG-IgG titers (may indicate higher disease activity) 3

Most commonly used agents 2, 8:

• Rituximab (B-cell depletion)
• Azathioprine
• Mycophenolate mofetil
• Monthly IVIG

Critical Treatment Pitfall

Some MS disease-modifying therapies may be ineffective or harmful in MOG-EM due to different immunopathogenesis 1. This is why accurate diagnosis matters—misclassifying MOG-EM as MS leads to inappropriate treatment.

Monitoring Disease Activity

While MOG-IgG titers correlate with disease activity, routine titer monitoring for relapse prediction cannot currently be recommended due to insufficient data on predictive intervals and the fact that titers may rise only very shortly before attacks 1. However, persistently high titers, intrathecal MOG-IgG production, and elevated CSF IL-6 or complement C5b-9 may indicate higher disease activity and worse prognosis 3.

Key Diagnostic Algorithm Summary

1. Clinical presentation suggests MOG-EM (optic neuritis, LETM, ADEM, brainstem syndrome)
2. Order serum MOG-IgG using cell-based assay during acute attack if possible
3. If positive: Check for "red flags" that suggest alternative diagnosis
4. If low-positive: Ensure supportive clinical/MRI features present; consider second assay
5. If negative but high suspicion: Retest during acute attack or after treatment washout
6. Confirm diagnosis requires compatible phenotype + positive MOG-IgG + exclusion of better alternatives 5
7. Treat acutely with IVMP and prolonged taper; escalate to plasma exchange/IVIG if needed
8. Long-term therapy only for relapsing disease or severe monophasic events with poor recovery