Diagnostic Approach to Subarachnoid Hemorrhage

Initial Evaluation

Non-contrast CT of the head is the best initial study to rule out subarachnoid hemorrhage, and when performed within 6 hours of symptom onset using modern high-quality scanners, a negative result is sufficient to exclude SAH without requiring lumbar puncture, as recommended by the American College of Radiology 1
The American College of Radiology specifies that equipment must meet their specifications for high-quality CT scanners, which have demonstrated sensitivity of 98.7-100% in the first 6 hours after symptom onset 1

Within 6 hours of symptom onset, non-contrast head CT alone is sufficient when performed on high-quality scanners and interpreted by experienced radiologists, with a sensitivity approaching 98.7-100%, as stated by the American Heart Association 1
Beyond 6 hours from symptom onset, lumbar puncture for xanthochromia evaluation becomes necessary if CT is negative or non-diagnostic, with spectrophotometric analysis having 100% sensitivity and 95.2% specificity when combined with negative CT, according to the American Academy of Neurology 1

The 6-hour rule does not apply to patients presenting with atypical features, such as primary neck pain without headache, syncope as presenting symptom, new seizure, or new focal neurological deficits, who require full workup regardless of timing, as recommended by the American College of Emergency Physicians 1
The Ottawa SAH Rule identifies low-risk patients who may not need any imaging, but this rule has very low specificity, so most patients still require imaging, according to the Canadian Medical Association 1, 2

Once SAH is confirmed on non-contrast CT, CTA is the preferred next step to identify the bleeding source, with 96.5% sensitivity and 88% specificity for aneurysms overall, as stated by the American College of Radiology 3, 1
DSA with 3D rotational angiography remains the gold standard and should be performed when CTA is negative but SAH pattern suggests aneurysmal source, especially for small aneurysms <3 mm, according to the Society of NeuroInterventional Surgery 4, 5