Blood tests useful include neuronal autoantibody panels (serum and CSF)—anti‑NMDA receptor, LGI1, CASPR2, AMPAR, GABA‑B—and paraneoplastic antibodies (Hu, Yo, Ri, Ma2). Routine labs: CBC, electrolytes, liver/renal function, glucose, thyroid tests, and inflammatory markers (ESR, CRP). General autoimmune screens (ANA, ANCA) and tumour markers may also be checked to identify triggers.

Autoimmune encephalitis is diagnosed by subacute onset (within 3 months) of working‑memory loss, altered mental status or psychiatric symptoms, seizures or new focal deficits; supportive tests (medial‑temporal or other MRI abnormalities, CSF pleocytosis/oligoclonal bands, or EEG encephalopathic slowing/epileptiform activity); detection of neuronal autoantibodies confirms diagnosis; and alternative infectious, metabolic, toxic or neoplastic causes must be excluded.

A CSF autoimmune profile is a panel of cerebrospinal fluid tests that detect autoantibodies and inflammatory markers linked to autoimmune neurological disorders. It typically examines neuronal surface and intracellular antibodies (e.g., NMDAR, LGI1, CASPR2), oligoclonal bands, IgG index, and general CSF cell count/protein. Results support diagnosis of autoimmune encephalitis or other CNS autoimmune diseases and guide immunotherapy and further investigation.

Key biomarkers for autoimmune encephalitis include neuronal autoantibodies in CSF or serum (cell‑surface: anti‑NMDA, LGI1, CASPR2, AMPAR, GABA‑B; intracellular: GAD65, anti‑Hu), CSF inflammatory changes (lymphocytic pleocytosis, elevated protein, oligoclonal bands), MRI limbic hyperintensities, characteristic EEG patterns (e.g., extreme delta brush), PET abnormalities, and serum–CSF antibody concordance; response to immunotherapy supports diagnosis.

Diagnosis of encephalitis most accurately relies on lumbar puncture with cerebrospinal fluid (CSF) analysis, including PCR testing for common viral pathogens (eg, HSV), cell count, glucose and protein. CSF PCR provides high sensitivity and specificity for viral causes. MRI brain is the most sensitive imaging test to detect inflammation and rule out alternative diagnoses, and EEG can support assessment of encephalopathic changes.

Autoimmune encephalitis is often not visible on CT scans; CT has low sensitivity and frequently appears normal or shows nonspecific swelling. MRI is the preferred imaging — it may reveal T2/FLAIR hyperintensity in the medial temporal lobes or limbic structures. Diagnosis also relies on lumbar puncture, EEG and antibody testing. Early recognition and treatment improve outcomes.