In suspected subarachnoid hemorrhage with a negative noncontrast CT, which CSF finding confirms the diagnosis?

• A. Decreased protein with xanthochromia
• B. Elevated white blood cell count
• C. Elevated red blood cell count that does not clear from the first to the fourth tube
• D. Increased glucose level

Answer: (C)

When subarachnoid hemorrhage is suspected but the head CT is negative, CSF analysis after lumbar puncture is used to confirm血. The most convincing clue is red blood cells present in the CSF that do not clear across sequential tubes (they remain elevated from the first to the fourth tube). This pattern distinguishes a true subarachnoid bleed from a traumatic puncture, where the RBC count typically decreases from the first tube onward as the sample is cleaned. Xanthochromia can support the diagnosis, but it may take time to develop, whereas persistent red blood cells across all samples provides a more immediate confirmation. Elevated white blood cells or abnormal glucose levels are nonspecific and do not confirm SAH.