How to tell a traumatic tap vs. SAH

18 Jan
–There is no criteria for how many RBCs in the CSF are needed to diagnose SAH
–One of the best methods to distinguish traumatic tap vs SAH is by looking for xanthochromia
–Can measure xanthochromia by visual inspection (subjective, human error) OR spectrophotometry (very sensitive but not very specific, not widely available at most hospitals)
–Occurs via breakdown of Hgb -> oxyhemoglobin (pink-orange, can happen in vitro) -> bilirubin (yellow, only happens in vivo)
False positive xanthochromia can occur from jaundice (usually total serum bili of at least 10-15 mg/dL), rifampin, high CSF protein concentration (>150 mg/dL), or excess carotenoid intake
–Oxyhemoglobin can be present in traumatic tap and appear faintly yellow
–Formation of bilirubin takes time, but after 12 hrs from onset of aneurysm rupture (i.e. “worst HA of my life”), CSF should show xanthochromia in patients with SAH
Elevated opening pressure (> 20 cm H2O) + bloody CSF strongly suggests SAH
–When all else fails, you may repeat the LP at a higher interspace
Submitted by F. DiFranco.

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