Abstract
Dear Sir,
We welcome the chance to respond to Dr Absalom's letter, which questions our failure in the revised national guidelines for cerebrospinal fluid (CSF) spectroscopy to recommend routine consideration of plasma bilirubin when increased CSF bilirubin with a visible oxyhaemoglobin absorbance peak are found. 1
Because the group was concerned that existing reporting practices tended to ‘underplay’ the likelihood of subarachnoid haemorrhage, two of the principal aims of the guideline revision were to emphasize the clinical significance of increased bilirubin with a visible oxyhaemoglobin absorbance peak, and to simplify reporting of results in these patients.
The group's decision not to recommend consideration of plasma bilirubin routinely in such patients was taken after considerable discussion and was based on pragmatism. At the time, we were of the opinion that the incidence of patients with raised CSF bilirubin owing to raised plasma bilirubin who also had oxyhaemoglobin present due to a traumatic tap was likely to be small. This view has subsequently been supported by unpublished data from Dr Robert Beetham indicating that only two out of a total of 2304 cases over a five-year period had net bilirubin absorbance >0.007 AU, net oxyhaemoglobin absorbance >0.02 AU and plasma bilirubin concentration >20 μmol/L. In some laboratories, spectroscopy results are reported by relatively inexperienced staff and plasma bilirubin results may not be easily available. Our priority was to ensure that if raised CSF bilirubin along with an oxyhaemoglobin absorbance peak were found, then the requesting clinician would be alerted to the very real possibility that the patient had had a subarachnoid haemorrhage. This is not overstating the significance of the findings. As Dr Absalom observes, clinicians are used to coping with uncertainty and understand that a comment stating ‘consistent with subarachnoid haemorrhage’ does not mean that a patient has definitely had a subarachnoid haemorrhage.
It is important to appreciate that these guidelines are advisory and cannot cover every eventuality. They are not a substitute for professional judgement and they explicitly state that ‘the final interpretation should take into account the available clinical information’. This would include any history of jaundice or a raised plasma bilirubin, in which case any correction for bilirubin would be at the discretion of the laboratory professional reporting the results. We agree with Dr Absalom that discussion with the requesting clinician is the ideal way of ensuring that all relevant clinical information is taken into account, but, unfortunately, this is unlikely to happen in laboratories where results are reported by inexperienced staff.
Yours faithfully,