A 70-year-old on NSAIDs presents with acute epigastric pain. Erect chest X-ray shows a thin crescentic lucency under the right hemi-diaphragm. CT abdomen confirms free air under the diaphragm. The erect CXR can detect free air as small as:

• A 10 mL of free intraperitoneal gas
• B 50 mL of free intraperitoneal gas
• C 1 mL of free intraperitoneal gas ✓
• D 100 mL of free intraperitoneal gas

Explanation

The erect chest X-ray is highly sensitive for pneumoperitoneum and can detect as little as 1 mL of free intraperitoneal gas as a subdiaphragmatic lucency, typically on the right (the right hemi-diaphragm being easier to distinguish from the hepatic gas shadow). CT is more sensitive than CXR but CXR remains the standard first-line imaging. Left-lateral decubitus view can also detect small amounts between the liver and the abdominal wall.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.