A study of almost 2,000 patients presenting with a sudden, acute headache was used to develop three rules to help ascertain whether a patient requires urgent investigation.
The rules are based on the patient's age, BP, the presence of neck pain or stiffness and whether the headache's onset accompanied physical exertion.
The new system could help prevent unnecessary investigations, said the authors of the study, conducted at a series of teaching hospitals across Canada.
Headache is a common presenting symptom in A&E and general practice. Coma and other neurological defects make certain cases of subarachnoid haemorrhage relatively straightforward to diagnose.
But around half of patients with subarachnoid haemorrhage present with an acute headache but are otherwise neurologically intact, said the paper's authors.
Lumbar puncture is traditionally carried out to exclude the diagnosis, even if results of a scan are negative. But the researchers said more work would be needed before the decision rules are clinically applicable.
'Further study of these proposed decision rules, including prospective validation, could allow clinicians to be more selective and accurate when investigating patients with headache,' they said.
An accompanying editorial in the BMJ said the findings are encouraging, and further work should consider the cost-effectiveness of various types of headache interventions.