Although a CT brain scan within 12 h of the onset of headache confirms the diagnosis in about 98% of subarachnoid haemorrhage (SAH) patients (Van der Wee et al. 1995), blood is visible in only about 50% a week later (Van Gijn & Van Dougen 1982). Clearly therefore some patients with a sudden onset headache and a normal CT brain scan may still have had an SAH, particularly if the scan is delayed. Detecting the final few cases is important because a missed aneurysmal SAH can be fatal if a recurrent bleed occurs, and clipping or coiling will prevent this.
The standard follow-on test after a negative CT is evidence of blood in the cerebrospinal fluid (CSF). Any test must be sensitive enough to pick up cases of aneurysmal SAH and so allow timely intervention, but specific enough not to expose too many patients to the risk of unnecessary tests,
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