CSF Antibody Testing for Limbic Encephalitis
When evaluating for limbic encephalitis, you should test CSF for a comprehensive neuronal antibody panel including both cell-surface antibodies (NMDAR, LGI1, CASPR2, AMPAR, GABA-B receptor, glycine receptor) and onconeuronal antibodies (Hu, Ma2, CV2/CRMP5, amphiphysin), along with anti-GAD antibodies. 1
Priority Testing Strategy
The most recent guideline 1 emphasizes testing neuronal antibody (NAA) panels in both CSF and serum because detection sensitivity varies by antibody type:
CSF-Preferential Antibodies (test CSF first):
- NMDAR antibodies - most sensitive in CSF
- GFAP antibodies - more sensitive in CSF 1
Serum-Preferential Antibodies (serum may suffice):
- LGI1 antibodies - more sensitive in serum
- Onconeuronal antibodies (Hu, Ma2, etc.) - more sensitive in serum
- AQP4 antibodies - more sensitive in serum 1
Antibodies to Include in Your Panel:
Cell-Surface/Synaptic Antibodies (generally treatment-responsive):
- NMDAR (N-methyl-D-aspartate receptor) 2
- LGI1 (leucine-rich glioma-inactivated 1) 2
- CASPR2 (contactin-associated protein 2) 2
- AMPAR (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor) 2
- GABA-B receptor 2, 3
- Glycine receptor 2
- mGluR5 (metabotropic glutamate receptor 5) 2
Intracellular/Onconeuronal Antibodies (paraneoplastic, poorer prognosis):
Other Important Antibodies:
- Anti-GAD65 (glutamic acid decarboxylase) - technically intracellular but behaves differently, not typically paraneoplastic 2, 4
Critical CSF Analysis Beyond Antibodies
Don't stop at antibody testing. The guideline 1 mandates comprehensive CSF workup including:
- Cell count with differential (lymphocytic pleocytosis 20-200 cells common, can reach 900)
- Protein, glucose, CSF/serum glucose ratio
- Oligoclonal bands (unmatched in serum) - present in 34% of cases but highly specific (96%) 5
- IgG index and synthesis rate
- HSV1/2 and VZV PCR (critical to exclude infectious mimics)
- Bacterial/fungal cultures when appropriate
- Cytology and flow cytometry
Important Clinical Caveats
Normal CSF does not exclude autoimmune encephalitis. 1 If clinical suspicion remains high based on MRI findings (mesial temporal abnormalities) or clinical syndrome, proceed with antibody testing even with normal routine CSF parameters.
Timing matters: Collect blood samples before administering IVIG or plasmapheresis to avoid false results 1.
Multiple antibodies can coexist in the same patient, so comprehensive panel testing is essential rather than targeted single-antibody testing 1.
The distinction between cell-surface antibodies (better prognosis, immunotherapy-responsive) versus onconeuronal antibodies (paraneoplastic, poorer outcomes) fundamentally guides both cancer screening intensity and treatment expectations 2, 4.