Method of Analysis

UK recommendations [64] strongly advocate the use of spectrophotometry to detect bilirubin in the CSF believing this to be the finding most suggestive of SAH [64,66]. However, Perry et al [67] found that each of 4 different spectrophotometric methods (including NEQAS) had an unacceptably high false positive rate. This led to renewed concerns about unnecessary invasive investigation [66] and the possibility of the detection of incidental aneurysm that, in the context of acute headache, may lead to patient anxiety, difficult management decisions and perhaps to unnecessary endovascular or surgical intervention.

In contrast, in the US, analysis for xanthochromia is performed almost exclusively by visual inspection of the CSF supernatant [68]. This technique has been demonstrated to be very specific for SAH [69] but with questionable sensitivity [70-72]. However, the largest prospective trial on 592 patients supports the practice of using visual CSF analysis, demonstrating a sensitivity of 100% (95%CI 94%-100%) for the diagnosis of SAH [73].

Learning Bite

There is concern that spectrophotometric analysis of CSF may lead to false positive results and unnecessary investigation whereas visual inspection for xanthochromia may lead to false negatives and missed cases of SAH [64,73].

Whatever the method of analysis, it is important the clinician has a clear understanding of the interpretation of the CT and CSF results and balances this against good clinical judgement [42].