Blood tests
Essential tests	FBC, U+E, liver function tests, C reactive protein, glucose, magnesium, calcium, phosphate
Clinical feature	Tests to consider
Evidence of sepsis	Blood cultures and urinalysis
Systemic features (rash, arthralgia etc), suggestive changes on MRI and CSF	Neuroinflammatory screen: ESR, antinuclear antibody, antineutrophil cytoplasmic antibody, antidouble-stranded DNA, antiextractible nuclear antigen, ACE level, serum complement, antiphospholipid antibody, anticardiolipin antibody and lupus anticoagulant
Risk factors in history	HIV, tuberculosis PCR/T-Spot
Clinical suspicion	Toxicology screening for drugs of abuse and alcohol Serum ammonia
Underlying malignancy	Paraneoplastic antibody screen, N-methyl-D-aspartate (NMDA) receptor and voltage-gated potassium channel antibodies
Cognitive or neuropsychiatric features, movement disorders	NMDA, potassium channel antibodies, thyroid antibodies
Neuroimaging
Almost all patients will require brain imaging. The main choice lies between MRI and CT, though other modalities may be needed.
CT scan of head	■ Quick, cheap and readily available ■ Avoids need for sedation ■ Demonstrates blood, abscess, tumours, oedema, infarcts, bony injuries and changes accompanying venous sinus thrombosis
MR scan of brain	■ Differentiates between acute and chronic ischaemia ■ Better imaging of posterior fossa and brainstem ■ Better demonstration of changes in encephalitis and inflammatory conditions ■ More likely to show smaller mass lesions and vascular malformations
CT or MR cerebral venogram	Consider with papilloedema and features of raised intracranial pressure, focal neurological signs
Lumbar puncture
We advise a low threshold for performing a lumbar puncture in patients with acute symptomatic seizures (providing no contraindications), particularly if there is no head trauma or metabolic derangement, and no clear aetiology demonstrated on examination or neuroimaging.
Opening pressure	Raised in; ■ Venous sinus thrombosis ■ Brain swelling – meningitis (especially cryptococcal), encephalitis, trauma
Cell count	■ >5 White cells mm-3 is abnormal ■ Neutrophils – consider bacterial meningitis ■ Lymphocytes – consider viral meningitis or encephalitis, mycobacterial, and fungal infection ■ Mixed or either cell type predominant in inflammatory conditions ■ Prior antimicrobial treatment may give a false negative result
Protein	Raised protein suggests active central nervous system (CNS) pathology, infection, inflammation, malignancy or haemorrhage
Glucose	Significantly reduced in bacterial infection (ratio with serum glucose <60% is abnormal)
Other infectious tests	Viral PCR Cryptococcal antigen India ink stain Ziehl–Neelsen stain, tuberculosis PCR and culture JC virus PCR
Cytology (+/- immunophenotyping)	Malignant cells
Xanthochromia	If subarachnoid haemorrhage suspected
EEG
Although non-specific (especially in the older people), the EEG can sometimes help: ■ distinguishing seizures from non-epileptic attacks ■ identifying non-convulsive status epilepticus ■ in showing diffuse slowing of background rhythms, a non-specific abnormality which may be seen postictally, following traumatic brain injury or CNS infection and accompanying any encephalopathy ■ in showing focal slowing or periodic lateralising epileptiform discharges, which occur with any localised pathology, including viral encephalitis, tumours and vascular events CSF, cerebrospinal fluid; FBC, full blood count; ESR, erythrocyte sedimentation rate.