Lumbar puncture confirms the diagnosis of SAH but should be reserved for patients in whom no blood can be detected by CT. LP is contraindicated if the patient presents with an impaired level of consciousness or with focal neurological signs. Usually, LP should be deferred until at least 12 hours after the onset of the headache, the exception being the rare patient with such an incomplete history that meningitis cannot be excluded.
The CSF should be centrifuged and the supernatant examined for xanthochromia - straw coloured fluid, due to the breakdown products of haemoglobin - detectable between 12 hours and 2 weeks after the event. The CSF can only be declared normal following spectophotometric analysis for xanthochromia.
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