• Blood glucose
• Plasma sodium, potassium, urea and creatinine
• Liver function tests, albumin and calcium
• Full blood count
• C-reactive protein
• Blood culture if temperature <36°C or >38°C
• Arterial blood gases and pH
• Chest X-ray
• ECG

With focal neurological signs

• CT head (with CT angiography if basilar artery occlusion possible)

With meningism

• CT head, then LP if no cause identified

If cause remains unidentified, consider

• CT/LP (if not yet done)
• Plasma osmolality (see Table 11.3)
• Toxicology screen (send serum (10 mL) and urine (50 mL)).
• Prothrombin time
• Arterial ammonia level
• Cortisol
• Thiamine/red cell transketolase
• Blood film
• Autoantibody screen (VGKC, NMDAR, GABA_BR, AMPAR, TPO, paraneoplastic, GQ1b)
• MRI brain
• EEG