added nbme12 tag b/c directly answers the Q:
A previously healthy 57-year-old man comes to the emergency department 1 hour after the sudden onset of severe occipital headache. His temperature is 37.2°C (99°F), pulse is 92/min, respirations are 18/min, and blood pressure is 180/98 mm Hg. He is mildly lethargic but oriented to person, place, and time. Neurologic examination shows no other abnormalities. A CT scan of the head shows no abnormalities. Which of the following is the most appropriate next step in management?
Ans: Cerebrospinal fluid analysis
Explanation: Noncontrast CT scan of the head has a high but not perfect rate of sensitivity for the
detection of subarachnoid hemorrhage (SAH). Patients with a presenting history concerning for SAH who have a negative CT scan should undergo emergent lumbar puncture for cerebrospinal fluid analysis, which may reveal xanthochromia, increased opening pressure, and/or a persistently increased cerebrospinal fluid concentration of red blood cells.