Which initial investigations should be performed in a patient suspected of having a subarachnoid haemorrhage?

Perform a non-contrast CT head scan urgently if a person presents with suspected subarachnoid haemorrhage, especially within 6 hours of symptom onset, as its diagnostic accuracy is highest during this period NICE NG228.

If the CT scan done within 6 hours shows no evidence of haemorrhage, consider a lumbar puncture if the suspicion remains high, ideally after at least 12 hours from symptom onset, to detect xanthochromia (elevated bilirubin) in cerebrospinal fluid NICE NG228.

In cases where the CT scan is performed more than 6 hours after symptom onset and shows no haemorrhage, a lumbar puncture should be considered to confirm or exclude subarachnoid haemorrhage NICE NG228.