OBJECTIVES: This study identified the proportion of spontaneous subarachnoid haemorrhage (SAH) patients diagnosed by Lumbar Puncture (LP). Furthermore reporting the incidence of aneurysmal SAH if a CT scan performed within 6 h was reported as negative, and finally investigated if there has been a change in practice since the new NICE guidance for the diagnosis of SAH was published in November 2022. METHODS: A pragmatic multicentre audit was conducted in the UK and Ireland capturing referrals to 25 Neurosurgical centres between 1st November 2020-31st October 2023. Case referral identification was done in each unit using local medical records and referral databases based on local protocols. RESULTS: 10,187 cases of spontaneous SAH were diagnosed within the study period: 9,357 were diagnosed by CT and 717 by LP. 7% of all confirmed SAH cases underwent lumbar punctures to return a diagnosis of spontaneous SAH when a CT head scan was non-diagnostic. This yielded 213 (3%) diagnoses of aneurysmal SAH. 55 cases(1%) of aneurysmal SAH initially had negative CT head scans within 6 h of ictus and a positive LP. We did not identify any evidence of a change in practice following the introduction of the NICE guidance in November 2022. CONCLUSION: This study shows that LP continues to be an important diagnostic test that will confirm a diagnosis of aneurysmal SAH in a small, but significant number of patients with thunderclap headache. We provide new data that may impact the current NICE guidelines on the diagnosis of SAH.