What is the role of cerebrospinal fluid ferritin in the diagnosis of subarachnoid haemorrhage in computed tomography-negative patients?

BACKGROUND: Spectrophotometry of cerebrospinal fluid (CSF) for bilirubin is the recommended method for investigation in suspected cases of subarachnoid haemorrhage (SAH), when a computed tomography (CT) of the head is negative for blood. There is a potential need for a simpler alternative. Measurement of CSF ferritin might fulfil this need.
METHOD: We have measured ferritin in the CSF from 252 patients with suspected SAH who were negative on a CT of the head for blood, recruited on a consecutive intention to recruit basis from four centres. CSF spectrophotometry was performed on all samples. A positive outcome was taken as an aneurysm found on angiography that was treated or a discharge diagnosis of non-aneurysmal SAH.
RESULTS: A final diagnosis of aneurysmal SAH was made in six patients, an arteriovenous malformation in one and non-aneurysmal SAH in nine. Receiver operating characteristic (ROC) analysis showed that at 6.4 microg/L, sensitivity, specificity, positive and negative predictive values were 1.0, 0.48, 0.12 and 1.0, respectively. At 12 microg/L, these values were 0.81, 0.91, 0.38 and 0.98, respectively.
CONCLUSIONS: At an appropriate negative predictive value (1.0) for a rule-out test, ferritin has too low a specificity to function as a stand-alone test and we cannot recommend it as an initial screen to be followed by spectrophotometry.