How to tell a traumatic tap vs. SAH

QUICK REVIEW:

--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)

PEARLS:

--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.

Reference(s): Shah, K.H., & Edlow, J.A. Distinguishing traumatic lumbar puncture from true subarachnoid hemorrhage. The Journal of Emergency Medicine. 23(1), 67-74, 2002. picture