Current status of laboratory diagnosis of subarachnoid haemorrhage in Northern Ireland

Studies have shown that computed tomography (CT) scanning is 95% positive in patients scanned within 1 day of a suspected subarachnoid haemorrhage (SAH). However, generalized cerebral oedema can cause false-positive diagnosis of SAH on CT, while it is estimated that up to 2% of patients with SAH can have negative scans as a result of the small amounts of subarachnoid blood involved. Because of the risks involved, there is a reluctance to identify aneurysms in CT-negative patients by cerebral angiography without the supportive evidence of spectrophotometric examination of cerebrospinal fluid (CSF). CSF spectrophotometry is widely regarded as the best technique available for diagnosis of SAH and has replaced the visual inspection of CSF for xanthochromia that many accept as being inadequate. The test is now being carried out in an increasing number of centres throughout the UK and, in Northern Ireland, five hospital laboratories are involved in monitoring its performance by regional audit and a pilot NEQAS scheme. These have identified the limitations of CSF spectrophotometry, one of which is the time it takes for blood pigments to appear in the CSF, and has standardized issues of specimen requirements, laboratory procedures and the level of interpretation when reporting results.