Lumbar puncture and cerebrospinal fluid (CSF) analysis

What it is

CSF sampling via needle insertion into the subarachnoid space, typically L3/4 or L4/5 (below the spinal cord terminus at L1/2 in adults) - diagnostic (infection, inflammation, malignancy, subarachnoid haemorrhage) and therapeutic (pressure relief)

Key indications: suspected meningitis/encephalitis, subarachnoid haemorrhage (CT-negative), demyelinating disease workup, idiopathic intracranial hypertension, malignant/leptomeningeal disease, normal pressure hydrocephalus

Finding	Bacterial meningitis	Viral meningitis	Normal
Opening pressure	Raised	Normal/mildly raised	10-20cmH2O
WCC	High, neutrophil-predominant	Moderate, lymphocyte-predominant	<5/microL
Protein	High	Mildly-moderately raised	0.15-0.45 g/L
Glucose (CSF:serum ratio)	Low (<0.4)	Normal	~0.6

CT brain before LP if: reduced GCS, focal neurological signs, papilloedema, immunocompromise, or new seizure - to exclude a mass lesion/raised ICP risking coning
Xanthochromia (spectrophotometry, ideally sampled >=12 hours after headache onset) distinguishes true subarachnoid haemorrhage from a traumatic tap when CT is negative
Traumatic tap: RBC count falls between tube 1 and tube 4; xanthochromia is absent if truly traumatic and CT is negative

Contraindications: raised ICP with risk of coning (mass lesion, midline shift), coagulopathy/anticoagulation, local infection at the site, spinal cord compression
Post-LP headache (postural, worse upright) is the most common complication - managed conservatively (hydration, analgesia, caffeine); epidural blood patch if persistent
Therapeutic LP (large-volume CSF removal) is used in idiopathic intracranial hypertension for symptomatic relief and pressure measurement

Bacterial/viral/fungal/TB meningitis, subarachnoid haemorrhage, multiple sclerosis (oligoclonal bands), Guillain-Barre syndrome (albuminocytologic dissociation - raised protein, normal cell count), idiopathic intracranial hypertension

Post-LP headache typically resolves within days regardless of treatment; persistent headache beyond ~2 weeks warrants an epidural blood patch or reassessment for an alternative cause

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