diagnose a subarachnoid haemorrhage if the non-contrast CT head scan shows blood in the subarachnoid space (1)
a CT scan: the investigation of choice:
if positive, perform angiography
if negative, consider MRI if available or lumbar puncture
if a CT head scan done within 6 hours of symptom onset and reported and documented by a radiologist shows no evidence of a subarachnoid haemorrhage:
do not routinely offer a lumbar puncture
think about alternative diagnoses and seek advice from a specialist
if a CT head scan done more than 6 hours after symptom onset shows no evidence of a subarachnoid haemorrhage, consider a lumbar puncture
lumbar puncture is considered if CT scan is normal - a lumbar puncture must not be performed if there are features of raised intracranial pressure. If performed within 6-12 hours then CSF is uniformly blood-stained. If performed between 12 hours and 2 weeks after initial headache then the supernatant is xanthochromic
NICE state that allow at least 12 hours after symptom onset before doing a lumbar puncture to diagnose a subarachnoid haemorrhage (1)
if the CT head scan is done more than 6 hours after symptom onset, the evidence showed that diagnostic accuracy is reduced and false-negative results are more likely
when a lumbar puncture is indicated, then it should be done at least 12 hours after symptom onset, when bilirubin formation is sufficient to be detected reliably
diagnose a subarachnoid haemorrhage if the lumbar puncture sample shows evidence of elevated bilirubin (xanthochromia) on spectrophotometry
think about alternative diagnoses if the lumbar puncture sample shows no evidence of elevated bilirubin (xanthochromia) on spectrophotometry
attribute to a non-haemorrhagic "thunderclap" headache if both CT and CSF are negative
if angiography is negative despite a xanthochromic CSF, then MRI is indicated
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