Background: Between 2·5% and 5% of cases of subarachnoid haemorrhage
(SAH) give negative results for computed tomography (CT) scanning. Recent UK
guidelines make recommendations as to standardization of laboratory methodology for
the detection of SAH in individuals with a negative CT scan. We have compared this
approach with our routine assessment using first-order derivative spectrophotometry,
a recently reported iterative computer prediction model and measurement of
cerebrospinal fluid (CSF) ferritin concentration. All methods were judged against
cerebral angiography as the definitive means of determining patient outcome.
Methods: Scanning spectrophotometry of CSF is necessary for both the UK
guidelines and our in-house method. Absorbance measurements at 360, 405, 414 and 455
nm are required for the computer model. CSF ferritin concentration was measured using
a serum method on a DPC Immulite 2000. This gave a value of <12 µg/L for normal
CSF. Ethical approval and informed patient consent to additional investigation were
obtained.
Results: The sensitivity and specificity for the UK guidelines method,
derivative spectrophotometry (in-house), iterative computer model and CSF ferritin
were, respectively: 0·8, 1·0; 0·9, 1·0; 0·9, 0·5; 1·0, 0·78.
Conclusion: First-order derivative spectrophotometry had best
concordance with angiography. Combination of this with CSF ferritin measurement may
improve sensitivity, although the zero-order method recommended in the UK guidelines
for the detection of an intracranial bleed is more amenable and has comparable
performance.
