Petzold, A

Sharpe, LT

English

UCL Discovery

Letter to the Editor — “Visual assessment for xanthochromia needs norevisitation”Sir, recently Linn et al. compared visual with spectrophotometric as-sessment of xanthochromic cerebrospinal ﬂuid (CSF) and concluded that asample perceived as colourless is incompatible with the diagnosis of recentsubarachnoid haemorrhage (SAH) [1]. Six CSF samples spiked with bilirubinand one normal CSF sample were assessed by 102 subjects visually. Spec-trophotometry between 450–460 nm established extinctions from 0.01 to 0.09in the six spiked samples.The observations made by Linn et al. are valuable, albeit our interpreta-tion of their results diﬀers from their own. We think that the study conﬁrmsthat visual assessment of the CSF for low concentrations of bilirubin is in-ferior to spectrophotometry. To illustrate this point we compare in Table 1our own results contrasting visual with spectrophotometric detection of lowconcentrations of bilirubin with those of Linn et al. [1, 2]. For this post–hocanalysis, we have only included the observed frequencies of their samplesseen as “yellow” or as “colourless”. Samples for which no decision could bemade because the visual inspection was “doubtful” are not included. Clearly,both sets of data demonstrate that human colour vision is less sensitive thanspectrophotometry for detecting small amounts of bilirubin from the CSF. Itwould be interesting to analyse the original absorption spectra of the Linnet al. [1] to determine whether their visual threshold (extinction of 0.06)1corresponds to ours (excitation purity or saturation of about 2.4%) [2].We would like to pose three additional questions: (1) Why was an ar-bitrary cutoﬀ of 0.05 chosen as opposed to 0.023 which was the criterionpublished by the same group previously [3]? (2) Why was a spectral band-width (450–460 nm) selected for measuring an extinction as opposed to therecommended absorbance at 476 nm [4]? (3) Given that the major “reallife” problem for visual assessment of CSF from a recent SAH is likely tobe contamination with blood (of red colour) [5], why was visual assessmentrestricted to a sensitivity problem (i.e., detection of low concentrations ofbilirubin in an otherwise colourless sample)? If one extrapolates from pre-vious data [3] such an analysis is more appropriate for CSF samples takenquite some time after a SAH, rather than, as the authors intended, recentlyfollowing a SAH.References[1] F.H. Linn, H.A. Voorbij, G.J. Rinkel, A. Algra, and J. van Gijn. Visual in-spection versus spectrophotometry in detecting bilirubin in cerebrospinalﬂuid. J Neurol Neurosurg Psychiatry, 76:1452–1404, 2005.[2] A. Petzold, G. Keir, and T.L. Sharpe. Why human color vision cannotreliably detect cerebrospinal ﬂuid xanthochromia. Stroke, 36:1295–1207,2005.[3] M. Vermeulen, D. Hasan, B.G. Blijenberg, A. Hijdra, and J. van Gijn.2Xanthochromia after subarachnoid haemorrhage needs no revisitation. JNeurol Neurosurg Psychiatry, 52:826–808, 1989.[4] UK NEQAS. National guidelines for analysis of cerebrospinal ﬂuid forbilirubin in suspected subarachnoid haemorrhage. Ann Clin Biochem,40:481–408, 2003.[5] A Petzold, G Keir, and LT Sharpe. Spectrophotometry for xanthochro-mia. N Engl J Med, 351:1695–1696, 2004.Axel Petzold MD PhDDepartment of Neuroimmunology, IONThe Tavistock Intensive Care Unit, NHNNQueen SquareLondon WC1N 3BG, United KingdomTel.: +44 207837 3611 x4204Fax: +44 207837 8553E-mail: a.petzold@ion.ucl.ac.ukLindsay T. Sharpe PhDInstitute of Ophthalmology11-43 Bath StreetLondon EC1V 9EL, United Kingdom3Table1:Humancolourvisionislesssensitivethanspectrophotom-etryfordetectingsmallamountsofbilirubinfromtheCSF(datafromreferences[2,1]).Petzoldetal.Linnetal.ExcitationColourvisionSpectrophotometryχ2PExtinctionColourvisionSpectrophotometryχ2Ppurity(%)(observedfrequency)(observedfrequency)(at450-460nm)(observedfrequency)(observedfrequency)36.61.001.00...NS————18.21.001.00...NS————9.31.001.00...NS0.091.001.00...NSa4.80.901.00...NS0.071.001.00...NS2.40.451.008.25<0.010.060.951.004.67<0.051.660.001.0022.0<0.0010.040.721.0031.7<0.00011.040.001.0022.0<0.0010.020.091.00157.9<0.00010.620.001.0022.0<0.0010.010.121.00148.1<0.0001aThestatisticsarebasedonthenumbersfromTable1inreference[1]:foranextinctionof0.09noneofthecliniciansormedicalstudentssawthesampleascolourless(n=0),50cliniciansand49studentssawthesampleasyellow(n=99),and1clinicianand2studentswereundecidedandthereforeleftoutofthisanalysis.Incontrastspectrophotometryclearlyrevealedthepresenceofbilirubinandallcliniciansandstudentswouldhaveinterpretedthespectrophotometrictraceinthesameway(n=102).Thenumbersfortheotherextinctionswereanalysedanalogously.

2Xanthochromia after subarachnoid haemorrhage needs no revisitation.

National guidelines for analysis of cerebrospinal ﬂuid for bilirubin in suspected subarachnoid haemorrhage.

Spectrophotometry for xanthochromia.

Visual inspection versus spectrophotometry in detecting bilirubin in cerebrospinal ﬂuid.

Why human color vision cannot reliably detect cerebrospinal ﬂuid xanthochromia.

Visual assessment for xanthochromia needs no revisitation

Visual assessment for xanthochromia needs no revisitation

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