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 ¹.

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 ¹.

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 ¹.