In suspected pneumoperitoneum, which diagnostic finding is most sensitive for detecting free intraperitoneal air?

• A. CT scan showing free air
• B. Free air under the diaphragm on an upright chest X-ray
• C. Ultrasound demonstrating intraperitoneal air
• D. Abdominal X-ray without upright view

Answer: (B)

The key idea is how sensitive each imaging method is at picking up even small amounts of free air in the peritoneal cavity. CT scanning provides cross‑sectional, high-contrast images that can visualize very tiny volumes of intraperitoneal air, making it the most sensitive way to detect pneumoperitoneum. Air appears as dark pockets that CT can delineate even when amounts are minimal or when bowel gas obscures other views.

Upright chest X-ray can reveal free air under the diaphragm and is useful, especially when CT isn’t available, but it misses smaller or early amounts of intraperitoneal air and its sensitivity is lower than CT. An abdominal X-ray without an upright view is less reliable because free air may be hidden depending on patient position and bowel gas. Ultrasound can sometimes detect air, but its findings are less consistent and highly operator-dependent.

So, the finding most sensitive for detecting free intraperitoneal air is air visible on CT imaging.