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Imprecision of cerebrospinal fluid net bilirubin absorbance.

Analysis of cerebrospinal fluid (CSF) has an important role to play in the diagnosis of subarachnoid hemorrhage (SAH) (1). Computed tomography, the first line of investigation for suspected SAH, shows a decrease in diagnostic accuracy as the time interval between the suspected hemorrhage and the time of investigation increases. A computed tomography scan would be positive in 98% of patients presenting within 12 h after an event, but positivity decreases to ∼50% in patients presenting after 1 week, 30% after 2 weeks, and 0% after 3 weeks (2).

An increase in bilirubin in the CSF is the key finding supporting the occurrence of SAH (3). Guidelines for the analysis of CSF for bilirubin in suspected SAH have recently been published in the United Kingdom (3). The presence of bilirubin is assessed by calculating the net bilirubin absorbance (NBA) according to Chalmers’ modification (4) to the original method of Chalmers and Kiley (5). A single NBA cut point of 0.007 absorbance units (AU) is recommended in the decision tree for interpretation and reporting of results (3). The interpretative comment “No evidence to support SAH” is advised if the NBA value is ≤0.007 AU and no oxyhemoglobin is detected. In contrast, if the corrected NBA is >0.007 AU and no oxyhemoglobin is detected, the interpretative comment “Consistent with SAH” is recommended (3).

Laboratories and clinicians need to define and be aware of the quality of the tests they provide when these tests are the basis for clinical decisions. To the best of our knowledge, the analytical imprecision profile for NBA has not been published to date. Our laboratory has performed 92 analyses of CSF for suspected SAH in the last year. Of …