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Papilledema, focal neurologic signs, immunocompromised patients, known mass lesion are reasons to get this test prior to a lumbar puncture
What is CT of the head prior to LP.
In patients who do not present with these signs, have normal mental status, and have no focal neurologic deficits, a head CT is not always required prior to LP. However, in most patients who have a clinical presentation consistent with acute bacterial meningitis or encephalitis, there will be enough diagnostic uncertainty that CT is advisable prior to LP