For healthcare professionals

Ottawa SAH Rule

The Ottawa subarachnoid hemorrhage (SAH) rule helps decide which alert patients with a sudden severe headache need investigation for bleeding around the brain. It uses six findings from the history and examination. Perry and colleagues published it in 2013. This tool is written for clinicians and is not a diagnosis.

Last reviewed: 2026-11-02 By the uncovermyrisk.com editorial team Educational only, not medical advice.

A sudden, severe headache, or a headache with fainting, a seizure, confusion, weakness or a stiff neck: call 911 now.General safety advice (not part of the published rule). The rule is for patients already being assessed; it does not replace urgent care.

Patient findings

Tick each finding that applies. Nothing you enter leaves this page.

Does the rule apply?

Alert, over 15 years old, new severe non-traumatic headache reaching maximum intensity within 1 hour Any of: new neurologic deficit, previous aneurysm, SAH or brain tumor, or a history of similar headaches (3 or more episodes over 6 months or more)

The rule applies only if the first box is ticked and the second is not.

Findings

Symptoms of neck pain or stiffnessAge 40 years or olderWitnessed loss of consciousnessOnset during exertionThunderclap headache (peak intensity immediately)Limited neck flexion on examination

Result appears here

What you will see

Whether the rule calls for further investigation for SAH, and which findings triggered it.

Educational only, not medical advice. Not a diagnosis. Testing decisions belong to the treating team.

Source

Perry JJ and colleagues, JAMA 2013. The rule, its eligibility and exclusions are taken from the open-access (CC BY) Table 3 of Patel and colleagues, International Journal of Emergency Medicine 2021, and match the six items in Suzuki and colleagues, Scientific Reports 2021 (CC BY). The tests check each of the six findings, the eligibility box and the exclusion box.

Validated in

Alert patients over 15 years of age with a new severe non-traumatic headache that reached maximum intensity within 1 hour. Not for patients with new neurologic deficits, previous aneurysm, SAH or brain tumor, or a history of similar headaches (Patel 2021).

Limits

The rule is built to rule out SAH, not to find it: a finding means further investigation, not that SAH is present. Specificity is low; in a pooled analysis, strict use of the rule would send 76% of patients without SAH for further investigation (Walton 2022).

The rule

For an eligible patient, investigate further for SAH if one or more of the following is present (Patel 2021):

Symptoms of neck pain or stiffness
Age 40 years or older
Witnessed loss of consciousness
Onset during exertion
Thunderclap headache (peak intensity immediately)
Limited neck flexion on examination

If all six are absent, the patient is at low risk of SAH by the rule (Suzuki 2021). The rule was derived and validated by Perry and colleagues (Perry 2013).

Worked example: an alert 32-year-old whose severe headache built up over 20 minutes while sitting at a desk, with no neck pain, no fainting and full neck flexion, has none of the six findings, so the rule rates the patient low risk. If the same headache had started during exercise, the rule would call for further investigation.

What the studies found

A systematic review pooled 8 studies with 8,114 patients. The pooled sensitivity of the rule was 99.5% (95% confidence interval 90.8% to 100%) and the pooled specificity 23.7% (15.5% to 34.4%) (Walton 2022).

In a Japanese teaching hospital, the rule identified all 56 patients with SAH (sensitivity 100%). Thunderclap headache was the most frequent finding, present in 44 of 46 patients with data (Suzuki 2021).

Related calculators

For stroke, see the NIH Stroke Scale calculator. For head injury, see the Canadian CT Head Rule and the Glasgow Coma Scale calculator. Other bedside scores are listed under For healthcare professionals.

Sources

Patel S, Parikh A, Okorie ON. Subarachnoid hemorrhage in the emergency department. Int J Emerg Med. 2021;14(1):31. doi:10.1186/s12245-021-00353-w. PMID 33980142. Open access (CC BY 4.0). Table 3: the Ottawa SAH rule with eligibility and exclusions. pmc.ncbi.nlm.nih.gov/articles/PMC8117305/
Suzuki T, Itokazu D, Tokuda Y. External validation for sensitivity of the Ottawa subarachnoid hemorrhage rule in a Japanese tertiary teaching hospital. Sci Rep. 2021;11(1):16717. doi:10.1038/s41598-021-96320-9. PMID 34408235. Open access (CC BY 4.0). Six items, low risk if all absent; sensitivity 56/56 (100%); thunderclap headache 44/46. pmc.ncbi.nlm.nih.gov/articles/PMC8373882/
Walton M, Hodgson R, Eastwood A, et al. Management of patients presenting to the emergency department with sudden onset severe headache: systematic review of diagnostic accuracy studies. Emerg Med J. 2022;39(11):818-825. doi:10.1136/emermed-2021-211900. PMID 35361627. Open access (CC BY 4.0). Table 2: pooled (8 studies, 8,114 patients) sensitivity 99.5%, specificity 23.7%; 76% of SAH-negative patients investigated. pmc.ncbi.nlm.nih.gov/articles/PMC9613855/
Perry JJ, Stiell IG, Sivilotti ML, et al. Clinical decision rules to rule out subarachnoid hemorrhage for acute headache. JAMA. 2013;310(12):1248-1255. doi:10.1001/jama.2013.278018. PMID 24065011. Original paper (cited; the criteria used here are taken from Patel 2021, CC BY). pubmed.ncbi.nlm.nih.gov/24065011/

Written and maintained by the uncovermyrisk.com editorial team. We do not have a medical reviewer; every claim links to its primary source above. Last reviewed: 2026-11-02. Spotted an error? See About.