A 32-year-old otherwise healthy female presents to the ED with “the worst headache of my life.” She is actively vomiting and appears altered. A non-contrast CT is normal. You are still concerned about subarachnoid hemorrhage. According to current literature, what should be your threshold of headache duration after which to consider performing an LP?

A. 2 hours

B. 6 hours

C. 12 hours

D. 24 hours. 

Answer is B: 6 hours. CT is nearly 100% sensitive for subarachnoid hemorrhage if obtained within the first 6 hours of headache onset. After this period of time, the sensitivity of CT for SAH begins to decline. Although additional CTA imaging may be helpful, it cannot definitively rule out SAH. At this point, you should share your concerns and discuss the possibility of a lumbar puncture procedure with your patient. It should be noted that the timing of this has been a debated topic among EM physicians given that an LP is not risk-free. Regardless, you need to start questioning the sensitivity of CT for SAH after 6 hours in a patient with concerning symptoms.

http://www.tamingthesru.com/blog/diagnostics/whats-in-a-tap-csf-analysis