EXPRESS: Uninterpretable cerebrospinal fluid absorbance scans caused by antibiotic therapy.

Background The revised national guidelines for analysis of cerebrospinal fluid for bilirubin in suspected subarachnoid haemorrhage (UK) provide an objective means of assessing cerebrospinal fluid (CSF) samples to determine risk of subarachnoid haemorrhage (SAH). The guidelines are intended for general use, but samples rendered uninterpretable due to the presence of the antibiotic doxycycline have been described. Here, further cases of antibiotic- based interference, and their implications, are presented.Methods An archival search of CSF spectra performed at Hallands County Hospital Halmstad was performed for the years 2011 and 2016- 2019 in an attempt to locate instances of interference. Each case of suspected interference was further investigated with in vitro reproduction experiments as a means of confirmation and assessment of potential clinical impact.Results A total of 10 cases of CSF curve interference were discovered: 6 due to doxycycline, 3 due to metronidazole and 1 due to tetracycline. Interference due to the tetracycline class was revealed through in vitro experimentation to cause an apparent decrease in the sampleâs net bilirubin absorbance(NBA); the presence of xanthochromia on visual inspection was, however, conserved.Conclusions The problem of CSF absorbance curve interference might be more common than previously suspected. Due to the potential NBA- lowering effect of tetracyclines, the author recommends visual examination of CSF samples in every case.