The diagnosis of subarachnoid haemorrhage (SAH) is a 'needle in a haystack' phenomenon: it is estimated that of the 2% of ED visits that present with headache, around 2% will actually have a SAH. In 2010 the Ottawa crew came up with yet another high-sensitivity 'rule-out' tool that sought to reduce downstream investigation (CT and LP).

 

American Journal of Emergency Medicine, May 2018