In suspected subarachnoid hemorrhage with a negative head CT, which study helps establish the diagnosis?

• A. Complete blood count
• B. Lipid profile
• C. Lumbar puncture
• D. Electrocardiogram

Answer: (C)

When subarachnoid hemorrhage is suspected but the initial head CT is negative, you need a test that directly shows blood or its breakdown products in the cerebrospinal fluid. A lumbar puncture provides that answer. CT scans are most sensitive for SAH in the first hours after onset, but their sensitivity wanes with time. If suspicion remains after a negative CT, examining CSF can confirm bleeding.

The key findings from a lumbar puncture are red blood cells in the CSF or xanthochromia, which is a yellowish discoloration from bilirubin as blood products break down in the CSF. Xanthochromia usually appears several hours after the hemorrhage, helping to confirm SAH even if the initial bleed wasn’t seen on CT. Interpreting the sample also involves distinguishing a true SAH from a traumatic tap: persistent red blood cells across multiple collected CSF tubes or visible xanthochromia supports SAH, whereas a decreasing number of RBCs in successive tubes with no xanthochromia suggests the blood came from the tap rather than the subarachnoid space.

Other options like complete blood count, lipid profile, or electrocardiogram don’t diagnose SAH; they’re not direct tests for bleeding in the subarachnoid space.