In suspected subarachnoid hemorrhage, what diagnostic step is performed if noncontrast CT is negative?

• A. CT head alone is sufficient.
• B. MRI brain is more sensitive and should be done first.
• C. Lumbar puncture after negative CT to detect xanthochromia or RBCs.
• D. Obtain EEG.

Answer: (C)

When someone is suspected of having a subarachnoid hemorrhage, a noncontrast CT scan is done first because it quickly shows bleeding if it’s present. But if the CT is negative and clinical suspicion remains high, you still need to look for blood in the cerebrospinal fluid. The next best step is a lumbar puncture to analyze the CSF for evidence of SAH.

CSF analysis can reveal two important signs of SAH: red blood cells and xanthochromia. Red blood cells in the CSF indicate that blood has been in the subarachnoid space. Xanthochromia is a yellow coloration produced by the breakdown of red blood cells into bilirubin, which can take several hours to develop and thus can detect a bleed that the CT missed. Because a traumatic tap can introduce blood into CSF during LP, clinicians often compare RBCs across sequential CSF tubes or look for xanthochromia to confirm true SAH rather than a false positive from the tap.

MRI can be more sensitive in some scenarios, but it’s not the immediate next step in the acute ED setting after a negative CT, and EEG isn’t used to diagnose SAH.