2 Neuroscience2.3 Basic techniques in neuroscience
2.3.1.5 CSF in neuropsychiatry
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Written by Resident Doctor Committee · Published
New in the 2027 syllabus
The College added this topic for exams from January 2027.
What CSF can show
A lumbar puncture takes a sample of cerebrospinal fluid (CSF), the fluid around the brain and spinal cord. Use it to look for infection, inflammation or neurodegeneration when the clinical picture suggests an underlying neurological disorder.
Opening pressure, cells, protein and glucose each answer a different question. Always compare CSF glucose with a blood glucose taken at the same time (a paired sample).
Before the procedure, assess bleeding risk, infection at the puncture site and features suggesting dangerous pressure gradients inside the skull.
Clinical question | Useful CSF tests | How to interpret them |
|---|---|---|
Infective encephalitis | Cell count, protein, glucose, pathogen polymerase chain reaction (PCR) and culture | Choose organism-specific tests based on the presentation and exposure history |
Autoimmune encephalitis | Cells, protein, oligoclonal bands and neuronal antibodies | Read alongside serum testing, the clinical syndrome, magnetic resonance imaging and electroencephalography |
Uncertain Alzheimer disease | Amyloid-beta and tau biomarkers | A lower amyloid-beta 42 relative to 40, and raised phosphorylated tau, support Alzheimer pathology |
Rapidly progressive dementia | Targeted tests for infection, inflammation and prion disease | CSF real-time quaking-induced conversion (RT-QuIC) supports a prion diagnosis in the right clinical syndrome |
Limits of interpretation
A normal routine CSF result does not rule out autoimmune encephalitis. The clinical syndrome guides which antibodies to test for and how to read the result.
Where you test for antibodies matters. CSF testing is particularly informative in N-methyl-D-aspartate (NMDA) receptor encephalitis, and a positive result in serum alone can mislead.
Dementia biomarkers help identify the cause. They do not measure everyday function. Specialist testing helps most when settling the uncertainty would change management.
Research findings, such as monoamine metabolites and inflammatory markers, show associations across groups of patients. They are not routine diagnostic tests for common psychiatric disorders.
An isolated abnormal result is not proof of cause. First consider how well the assay performs, blood contamination, the patient's age and the pre-test probability (how likely the diagnosis was before testing).