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Over the past few years, there have been numerous studies discussing the "best" way to diagnose subarachnoid hemorrhage (SAH). These 2016 guidelines review the current evidence.
Classic approach: dry CT, and if negative perform the lumbar puncture (LP)
It is the most common approach, with the most robust evidence. Still considered "standard of care"
Dry CT alone: Sensitivity of a dry CT alone for SAH has increased with improved technology, and the sensitivity is highest when done within the first 6 hours of headache onset. Despite studies quoting a sensitivity of 100% within 6 hours, this evidence is still insufficient due to concerns for selection bias in the study, and the fact that the CTs in the study were read by neuroradiologists.
CT/CTA: CTA is very sensitive for aneurysmal SAH (98% for aneurysms >3mm). CTA would miss non-aneurysmal SAH, but would detect aneurysms that may or may not need to be treated before rupture. It is a reasonable strategy to exclude aneurysmal SAH in select patients, and in patients who refuse LPs or in whom the LP results are equivocal.
Bottom Line: CT/LP is still standard of care, with CT/CTA being an acceptable alternative if LP is equivocal or refused by the patient. CT alone is NOT enough to exclude SAH.
References
Clinical Guidelines for the Emergency Department Evaluation of Subarachnoid Hemorrhage. Meurer WJ, Walsh B, Vilke GM and Coyne CJ. JEM 2016; 50(4) 696-701