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The Ottawa Subarachnoid Hemorrhage Rule identifies patients with acute nontraumatic headache who require CT to rule out SAH. If any of six high-risk findings is present, CT is indicated; if none are present, CT is not required (100% sensitivity).
Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.
The Ottawa SAH Rule was derived and validated by Perry and colleagues in a multicenter Canadian cohort (JAMA 2013). Among patients presenting to the emergency department with acute nontraumatic headache peaking within 1 hour, the presence of any of six findings — age ≥ 40 years, neck pain or stiffness, witnessed loss of consciousness, headache onset during exertion, thunderclap headache (instantly peaking pain), or limited neck flexion on examination — identified all patients with subarachnoid hemorrhage (100% sensitivity) while being negative in 15.3% of patients without SAH. It is widely used as an ED rule-out tool for SAH.
Specialty
NeurologyCategory
NeurologyDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-08-16
Formula
Keywords
Normal Range
Rule negative (none present) = CT not required (sensitivity 100%, specificity 15.3%). Rule positive = CT indicated.
| Classification | Range |
|---|---|
Rule negative CT for SAH not required (100% sensitivity, 15.3% specificity) | 0 criteria |
Rule positive CT for SAH indicated | ≥ 1 criteria |
Determines whether an emergency department patient with an acute nontraumatic headache requires a non-contrast CT to rule out subarachnoid hemorrhage. The rule is positive if any of six high-risk findings is present and has 100% sensitivity for SAH.
The Ottawa SAH Rule counts the presence of six high-risk findings. A result of 0 of 6 is negative (CT not required to rule out subarachnoid hemorrhage); any single criterion present makes the rule positive and CT is indicated.
Age 40 years or older; Yes counts as one high-risk criterion.
Neck pain or stiffness; Yes counts as one criterion.
Loss of consciousness witnessed by another person; Yes counts as one criterion.
Headache that began during exertion; Yes counts as one criterion.
Abrupt, instantly peaking headache; Yes counts as one criterion.
Restricted neck flexion on examination; Yes counts as one criterion.
Rule negative (0 criteria): CT to rule out SAH is not required — sensitivity 100%, specificity 15.3%. Rule positive (≥ 1 criterion): non-contrast CT is indicated. A negative CT with ongoing suspicion may still require lumbar puncture per local protocol.
The Ottawa SAH Rule provides an evidence-based, highly sensitive method to rule out subarachnoid hemorrhage in ED patients with acute headache, avoiding unnecessary CTs while safely identifying those who need imaging.
A 58-year-old man presents with a sudden severe headache that peaked instantly while he was lifting weights, described as the worst headache of his life. His neurologic examination is normal.
Inputs:
Expected Result: Ottawa SAH Rule POSITIVE (3 of 6 criteria) — non-contrast CT is indicated to rule out SAH.
CT to rule out SAH is not required (100% sensitivity); other causes of headache should still be considered clinically.
Modern CT within 6 hours is highly sensitive; when CT is negative but suspicion persists, lumbar puncture may still be performed per local protocol.
Prognosis and treatment planning
Monitoring neurologic status
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. The Ottawa SAH Rule applies only to the specified population; always apply clinical judgment and local protocol.