Cerebrospinal fluid (CSF), particularly in the context of delirium or acute confusional state

Role

Lumbar puncture/CSF analysis is used selectively in the workup of an older patient with an acute confusional state (delirium) when a CNS infective, inflammatory, or haemorrhagic cause is suspected - not a routine or first-line test for delirium itself
Most delirium in older patients has a systemic (infection, metabolic, drug-related) rather than a primary CNS cause, and CSF analysis is reserved for cases where clinical features specifically raise suspicion of a CNS process

CNS infection (meningitis, encephalitis) can present atypically in older patients - with confusion alone, blunted fever response, and without the classic meningism seen in younger patients - raising the risk of the diagnosis being missed if LP is not considered when clinical suspicion exists

Indications for LP in this context: new confusion with fever/meningism, unexplained confusion with focal neurological signs or seizure, suspected CNS vasculitis/autoimmune encephalitis, or when standard first-line delirium workup (bloods, urinalysis, imaging) is unrevealing and suspicion of a CNS cause remains
CT brain before LP if reduced GCS, focal signs, or papilloedema, to exclude a mass lesion/raised ICP risk of coning - the same precaution as in younger patients

Treat the underlying cause identified (antimicrobial therapy for infection, immunosuppression for autoimmune/inflammatory causes) alongside standard supportive delirium management (orientation, sensory aids, avoiding precipitating drugs)
LP is not indicated as a routine test in delirium without specific clinical features suggesting a CNS cause - it should be targeted, not reflexive

Delirium, CNS infection in the elderly (atypical presentation), autoimmune encephalitis, subarachnoid haemorrhage as an occasional confusional-state mimic
Atypical presentation (afebrile, no meningism) of CNS infection in older adults is a recognised trap - a normal-appearing systemic workup does not exclude a CNS cause if the clinical picture remains otherwise unexplained

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