In suspected subarachnoid hemorrhage with a normal noncontrast CT scan, which diagnostic study is indicated next?

• A. Digital subtraction angiography
• B. Head CT scan with contrast
• C. Lumbar puncture
• D. Magnetic resonance angiography

Answer: (C)

When subarachnoid hemorrhage is strongly suspected but the initial noncontrast CT is normal, the next step is lumbar puncture to analyze the CSF for evidence of bleeding. CT is highly sensitive for acute SAH, especially within the first hours, but its ability to detect SAH wanes over time. If blood is present in the CSF or xanthochromia (yellow discoloration from bilirubin) is detected, it confirms SAH even when CT was negative. This CSF testing helps distinguish true SAH from other causes of headache when imaging doesn’t show blood.

Once SAH is confirmed, the usual next step is to identify the aneurysm or source with vascular imaging, but that confirmation step hinges on the SAH diagnosis from CSF. Other imaging like a head CT with contrast or MR angiography can be considered in different contexts, but they are not the immediate confirmatory test after a normal noncontrast CT in this scenario.