At present, evidence and opinion dictate that all patients presenting with acute severe headache <2 weeks from the index episode should undergo non-contrast CT scanning of the brain.
If this is reported as normal (ideally by a neuroradiologist) then an LP should be undertaken at least 12 hours from the start of the headache. The last CSF sample should be protected from the light and transported quickly to the laboratory for analysis (by spectrophotometry in the UK) for xanthochromia. If both CT and LP are negative within two weeks then SAH can be excluded. Patients presenting >2 weeks from the index headache or in whom results of either CT or LP have been unobtainable or dubious should be discussed with a neurosciences centre.