If a non-contrast CT does not show subarachnoid hemorrhage but SAH is still suspected, which diagnostic test can confirm the diagnosis?

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Multiple Choice

If a non-contrast CT does not show subarachnoid hemorrhage but SAH is still suspected, which diagnostic test can confirm the diagnosis?

When SAH is still suspected but the initial noncontrast CT is negative, lumbar puncture is the test that can confirm the diagnosis. Analyzing the CSF allows you to detect blood from the bleed that the CT may have missed and, more importantly, to look for xanthochromia—the yellow discoloration from bilirubin formed as red blood cells break down in the CSF. Xanthochromia confirms that blood has been present in the subarachnoid space, helping distinguish true hemorrhage from a traumatic tap. RBCs in CSF can also indicate SAH, but they can be seen with a traumatic tap, so xanthochromia provides stronger confirmation. MRI with contrast is less reliable acutely, and repeating CT after 24 hours is not as definitive as LP for confirming SAH. An electrolyte panel and vital signs alone won’t establish the diagnosis.

When SAH is still suspected but the initial noncontrast CT is negative, lumbar puncture is the test that can confirm the diagnosis. Analyzing the CSF allows you to detect blood from the bleed that the CT may have missed and, more importantly, to look for xanthochromia—the yellow discoloration from bilirubin formed as red blood cells break down in the CSF. Xanthochromia confirms that blood has been present in the subarachnoid space, helping distinguish true hemorrhage from a traumatic tap. RBCs in CSF can also indicate SAH, but they can be seen with a traumatic tap, so xanthochromia provides stronger confirmation. MRI with contrast is less reliable acutely, and repeating CT after 24 hours is not as definitive as LP for confirming SAH. An electrolyte panel and vital signs alone won’t establish the diagnosis.

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