Diagnostic Approach to Subarachnoid Hemorrhage

Initial Imaging

Non-contrast CT head is the best initial imaging test for suspected subarachnoid hemorrhage, and should be performed immediately when SAH is clinically suspected, as recommended by the American College of Radiology 1, 2, 3
The American College of Radiology specifically recommends against using contrast in the initial CT scan because the goal is to detect blood, not characterize vessels 1, 2, 5
CT has 98.7-99.9% sensitivity when performed within 6 hours of symptom onset, missing fewer than 1.5 in 1000 SAHs, according to the American Heart Association 1, 2, 5

Within 6 hours of headache onset: A negative high-quality CT scan interpreted by a neuroradiologist is likely sufficient to exclude SAH, and lumbar puncture may not be necessary, as stated by the American College of Emergency Physicians 1, 2, 5
Beyond 6 hours from symptom onset: CT sensitivity declines (93% at 24 hours, 57-85% at 6 days), making lumbar puncture mandatory if CT is negative but clinical suspicion remains high, according to the American Academy of Neurology 1, 2, 6

The Ottawa SAH Rule has 100% sensitivity but only 7.6-15.3% specificity, meaning it identifies a small subset who can safely avoid testing, but most patients meeting criteria will still need imaging, as recommended by the Canadian Medical Association 1, 2
Patients with acute severe headache reaching maximum intensity within 1 hour require investigation if they have ANY of: age ≥40 years, neck pain or stiffness, witnessed loss of consciousness, onset during exertion, thunderclap headache, or limited neck flexion on examination, according to the American College of Radiology 1, 2, 3

Once SAH is confirmed on non-contrast CT, proceed immediately to vascular imaging: CTA head is fast, non-invasive, and has >90% sensitivity for aneurysms, making it the appropriate next step, as recommended by the American College of Radiology 7
CTA sensitivity decreases for aneurysms <3mm (61-85% sensitivity), so catheter-based digital subtraction angiography (DSA) remains the gold standard with >98% sensitivity and specificity, according to the Society of Interventional Radiology 1, 7