Detection of xanthochromia in cerebrospinal fluid.

Lumbar puncture (LP) still has an important role to play in the diagnosis of subarachnoid hemorrhage. Although computed tomography (CT) scanning has replaced LP as the investigation of first choice, within 24 h of ictus 5% of cases will show no evidence of hemorrhage on CT scanning; this percentage is as high as 50% by 1 week, 30% after 2 weeks, and 0% after 3 weeks (1). By definition, xanthochromia is the yellow discoloration indicating the presence of bilirubin in the cerebrospinal fluid (CSF) and is used by some to differentiate in vivo hemorrhage from a traumatic LP. In contrast to CT, CSF xanthochromia is present in all patients up to 2 weeks postictus and is still present in 70% of patients at 3 weeks (1, 2). A minimum period for CSF bilirubin detection is 12 h postictus (2). Thus, the detection of bilirubin in CSF appears to be the test of choice at late time points. Spectrophotometry of CSF in the visible region is, in general, considered more sensitive than visual examination, with peaks at 415 and ;440–460 nm indicating the presence of hemoglobin (Hb) and bilirubin, respectively (1, 2). The problem with this test is that it is not known which CSF bilirubin absorbance indicates a clinically significant bleed. We have developed a simple quantitative method that attempts to address this question. CSF was collected aseptically by LP for routine biochemical and microbiological investigations. The supernatants from CSF specimens microcentrifuged at 13 000g for 1 min were scanned between 360 and 800 nm in a spectrophotometer (Model 7500, Beckman Instruments). In most CSF samples, volumes collected were ,500 mL and were scanned in 100-mL microcuvettes with a 1-cm light path. Volumes $500 mL were scanned in 1.5-mL cuvettes. After a scan was completed, the spectrum was autoscaled, and tangents were drawn from ;530 to 360 nm (Fig. 1). Perpendiculars to this tangent were measured in mm at 415 nm for Hb (a) and 440 nm for bilirubin (b). For pure bilirubin, this ratio of a/b was 1, whereas for Hb it was 8. In our scans, full-scale absorbance was represented by an 82-mm scale; hence the absorbance of Hb was a/82 multiplied by the full-scale absorbance, and the absorbance of bilirubin was b/82 multiplied by the fullscale absorbance. Because Hb increases the absorbance of bilirubin at 440 nm, the following correction to bilirubin absorption was made: