NMDA Receptor Antibody Positive Test
A positive NMDA receptor antibody test indicates autoimmune encephalitis and requires immediate initiation of immunotherapy with high-dose corticosteroids (or IVIG/plasmapheresis if steroids contraindicated), comprehensive cancer screening, and CSF confirmation since CSF testing has 100% sensitivity compared to only 85.6% for serum. 1, 2
What the Positive Test Means
A positive NMDA receptor antibody indicates antibody-mediated autoimmune encephalitis targeting the NR1 subunit of the NMDA receptor. CSF antibody testing is mandatory even if serum is positive, as CSF has superior sensitivity (100% vs 85.6% in serum) and specificity 2. Serum-only positivity without CSF confirmation should prompt immediate lumbar puncture, as approximately 14% of true cases are serum-negative 2.
Higher antibody titers correlate with worse outcomes and presence of teratoma, making quantification clinically useful 2.
Immediate Diagnostic Work-Up
Essential Testing (Complete Within 24-48 Hours)
Lumbar puncture is the highest priority:
- Test CSF for NMDA receptor antibodies (gold standard)
- Oligoclonal bands, IgG index, IgG synthesis rate
- Cell count to rule out infection before starting immunotherapy
- Basic studies to exclude CNS lymphoma/neurosarcoidosis 1
Brain MRI and EEG to identify focal/multifocal abnormalities 1
Cancer screening (initiate immediately, do not delay treatment):
- CT chest/abdomen/pelvis with contrast as initial screen
- Young adults with typical NMDAR presentation: transvaginal/transabdominal pelvic ultrasound for ovarian teratoma 1
- If initial CT negative and high cancer suspicion: add mammogram/breast MRI, and whole-body FDG-PET 1
Treatment Algorithm
First-Line Immunotherapy (Start Immediately After Ruling Out Infection)
For moderate presentations:
- High-dose IV methylprednisolone (1000mg daily × 3-5 days) 1
- If no improvement after completing initial cycle, add IVIG or plasmapheresis 1
For severe presentations (refractory status epilepticus, severe dysautonomia, rapidly progressive symptoms):
- Combination therapy from the start: corticosteroids PLUS IVIG or plasmapheresis 1
- This approach is preferred over sequential therapy in severe NMDAR encephalitis 1
IVIG vs Plasmapheresis selection:
- Choose IVIG first in: agitated patients, bleeding disorders 1
- Choose plasmapheresis first in: severe hyponatremia, high thromboembolism risk, cancer patients, associated demyelination 1
- Plasmapheresis: 5-10 sessions every other day 1
Second-Line Immunotherapy (If No Response After 2-4 Weeks)
Rituximab is the preferred second-line agent for NMDAR encephalitis (antibody-mediated) 1:
- Dosing: 375 mg/m² weekly × 4 weeks OR 1000mg × 2 doses (2 weeks apart) 1
- 80% of experts prefer rituximab over cyclophosphamide for antibody-mediated disease 1
Cyclophosphamide (600-1000 mg/m²) is reserved for cell-mediated autoimmunity (classical paraneoplastic syndromes with intracellular antibodies) 1
Refractory Cases (No Response 1-3 Months After Second-Line)
Consider escalation to:
- Tocilizumab (IL-6 inhibitor)
- Bortezomib (proteasome inhibitor)
- Note: Limited evidence for these agents 1
Maintenance Therapy
Bridging therapy after acute phase:
- Gradual oral prednisone taper OR
- Monthly IV methylprednisolone OR
- Monthly IVIG 1
Long-term maintenance (beyond 6 months):
- Generally not required for good responders 3
- Consider for: severe initial presentation, prolonged hospitalization, relapsing disease 3
- Options: rituximab redosing or mycophenolate mofetil 3
Critical Pitfalls to Avoid
Do not wait for antibody results to start treatment - CSF antibody results take 2-5 weeks; treat based on clinical suspicion once infection excluded 1
Do not rely on serum antibodies alone - 14% of true cases are serum-negative; always test CSF 2
Do not delay cancer screening - but do not delay immunotherapy waiting for screening results. Screen in parallel 1
Do not undertreate severe presentations - use combination first-line therapy from the start, not sequential 1
Monitor for ICU needs: refractory status epilepticus, severe dysautonomia, respiratory compromise, severe hyponatremia requiring controlled correction 1