What is known?
  • The Ottawa SAH rule is a clinical decision tool developed by Perry et al to identify patients at a high risk of SAH.

  • A review of the literature yielded eight studies validating the rule, demonstrating a sensitivity of 100% in all but one study and specificity ranging from 7.6-13.6%.

  • The Emerald SAH rule was derived by Kimura et al and uses objective, quantitative measures that aim to remove interobserver variability. It has not yet been externally validated.

What is the question?
  • To analyse the performance of the Ottawa and Emerald SAH rules in a contemporary UK cohort.

What was found?
  • This is the first external validation of the Ottawa SAH rule in a UK patient cohort.

  • Our findings strengthen the existing evidence by showing a high sensitivity at 100% for the rule, increasing confidence that the rule can be safely used to exclude SAH without further investigation. However, we again demonstrate that it is limited by low specificity at 22%.

  • This is the first ever external validation of the Emerald rule; it did not perform as well in this cohort and is unsuitable for clinical use.

What is the implication for practice now?
  • In patients presenting to the ED with an acute non-traumatic headache, the Ottawa rule can be used to safely exclude patients with SAH and can thereby inform decisions for further investigations.