Six hours means so much when it comes to subarachnoid hemorrhage. That is the inflection point when blood may no longer look like fresh blood on a CT. Several studies have shown that a negative ct done within 6 hours of headache onset effectively excludes clinically significant bleeding (extrapolated to mean aneurysmal subarachnoid hemorrhage). Of course there are other things that can cause severe acute headache in an otherwise healthy person: vascular dissection, dural sinus thrombosis to name a few. So it’s not all CT-LP, but focusing on the question of "LP yes or no" after a negative third generation (or higher) CT,  the evidence suggests that LP may not be mandatory. Can there still be bleeding? Yes, there can. But bleeding from a source that’s going to kill is unlikely. This is a time for shared decision making with your patient.

CT may miss a small number of bleeds that LP will find, but there is also the issue of a false positive tap being more likely than a bleed and the downstream effects of testing and aneurysm treatment.

Dr. Ran Ran joins the show to examine the literature on The Subarachnoid Enigma

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Blok, Katelijn M., et al. "CT within 6 hours of headache onset to rule out subarachnoid hemorrhage in nonacademic hospitals." Neurology (2015): 10-1212.

Perry, Jeffrey J., et al. "Differentiation between traumatic tap and aneurysmal subarachnoid hemorrhage: prospective cohort study." bmj 350 (2015): h568.

Perry, Jeffrey J., et al. "Sensitivity of computed tomography performed within six hours of onset of headache for diagnosis of subarachnoid haemorrhage: prospective cohort study." Bmj 343 (2011): d4277. 

Claveau, David, and Jerrald Dankoff. "Is lumbar puncture still needed in suspected subarachnoid hemorrhage after a negative head computed tomographic scan?." CJEM 16.3 (2014): 226-228.