• Michael D April, Samuel M Keim, Alex Koyfman, William J Meurer, Florian Schmitzberger, and Brit Long.
• 2(nd) Brigade Combat Team, 4(th) Infantry Division, Fort Carson, Colorado; Department of Military and Emergency Medicine, Uniformed Services University of the Health Sciences, Bethesda, Maryland.
• J Emerg Med. 2021 Jul 1; 61 (1): 97-104.

BackgroundAtraumatic subarachnoid hemorrhage (SAH) is a deadly condition that most commonly presents as acute, severe headache. Controversy exists concerning evaluation of SAH based on the time from onset of symptoms, specifically if the headache occurred > 6 h prior to patient presentation.Clinical QuestionDo patients undergoing evaluation for atraumatic SAH who have a negative computed tomography (CT) scan of the head obtained more than 6 h after symptom onset require a subsequent lumbar puncture to rule out the diagnosis?Evidence ReviewStudies retrieved included a retrospective cohort study, two prospective cohort studies, and a case-control study. These studies provide estimates of the diagnostic accuracy of head CT imaging obtained > 6 h from symptom onset and diagnostic test characteristics of subsequent lumbar puncture.ConclusionThe probability of SAH above which emergency clinicians should perform a lumbar puncture is 1.0%. This threshold is essentially the same as the estimated probability of SAH in patients with a negative head CT obtained more than 6 h from symptom onset. Emergency physicians might reasonably decide to either perform or forego this procedure. Consequently, we contend that the decision whether to perform lumbar puncture in these instances is an excellent candidate for shared decision-making.Copyright © 2021 Elsevier Inc. All rights reserved.

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