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Letters to the Editor |
Neurobiochemistry Laboratory, The Walton Centre for Neurology, and Neurosurgery, Lower Lane, Liverpool L9 7LJ, United Kingdom
aAuthor for correspondence: fax 44-151-5295576; e-mail danny.oconnell{at}thewaltoncentre.nhs.uk.
To the Editor:
A previous contribution to this journal (1) has rightly highlighted the difficulties associated with making a precise determination of heme pigments in suspected subarachnoid hemorrhage. In the United Kingdom, it is recommended that net bilirubin absorbance (NBA) is determined by zero-order spectrophotometry (2).
Viljoen et al. (1) reported large CVs at or near the cutoff of 0.007 absorbance units (AU; 22% at 0.0062 AU and 17% at 0.007 AU); their calculations were based on manual drawing of a predicted baseline and measurement of net absorbance by use of a ruler. This imprecision therefore has an impact on clinical decision-making. It is clear that positive NBA measurements calculated in this manner may lead to a high proportion of false-negative reports, with disastrous consequences, or, on the other hand, a high proportion of false positives, leading to unnecessary angiographic procedures used to locate the site of the aneurysm.
In our laboratory, we have demonstrated that the precision of NBA measurement is excellent at clinically relevant cutoff values if an appropriate spectrophotometer aided by computer software for analysis of scanned data is used.
We obtained cerebrospinal fluid (CSF) samples from patients with a negative or equivocal computed tomography scan by the standard lumbar puncture procedure. Samples were centrifuged for 5 min at 1831g as soon as possible after lumbar puncture. An undiluted CSF sample was scanned on a frequently serviced (6-month intervals) Unicam UV300 spectrophotometer (ThermoSpectronic) from 360 to 600 nm at a scan speed of 240 nm/min with a 2-nm bandwidth. Data collection points were set at 0.5-nm intervals. Pure water was used as blank along the entire wavelength range. Data were collected and subsequently analyzed with Vision 32 software (ThermoSpectronic). The use of this software allows simple postscan manipulation of the collected data. The zero-order trace was first subjected to high smoothing without loss of resolution. A predicted baseline was drawn by placement of cursors at the desired wavelengths with an accuracy of 0.5-nm steps.
CSF specimens used for the imprecision study were chosen to give a range of NBAs that might be encountered near the medical decision cutoff point of 0.007 AU. Eight CSF samples with a mean NBA in the range 0.00042–0.0973 AU were scanned 20 times.
The within-run imprecision was calculated across this range, and a precision profile was constructed (Fig. 1
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Computed NBAs gave CVs of 
1% near the cutoff point of 0.007 AU. This is a dramatic improvement over manual calculation. At all NBA values, the CVs were satisfactory down to 0.001 AU.
Obvious as it might seem, manual calculation of spectra could lead to misclassification of patients, with devastating consequences. For the sake of patient safety, we emphasize the need for spectrophotometric and spectral evaluation to be conducted with fit-for-purpose techniques.
References
The following articles in journals at HighWire Press have cited this article:
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  A Viljoen, K S Walker, C Ho, and P J Twomey Analysis of cerebrospinal fluid for suspected subarachnoid haemorrhage is improved by built-in spectrophotometer software. J. Clin. Pathol., June 1, 2006; 59(6): 667 - 668. [Full Text] [PDF]
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