On erect chest X-ray, free air under the diaphragm (pneumoperitoneum) appears as a crescent of gas. What is the minimum volume of free peritoneal gas that can be reliably detected on erect CXR?
- A 10–20 mL
- B 1–2 mL ✓
- C 50 mL
- D 100 mL
Explanation
Erect chest X-ray can detect as little as 1–2 mL of free intraperitoneal gas as a subphrenic crescent, making it the standard initial investigation for suspected perforation. The patient should ideally be erect for at least 5–10 minutes before the X-ray to allow gas to rise. Left lateral decubitus (patient's right side up) plain abdominal film is an alternative when the patient cannot stand, detecting gas between liver edge and right lateral abdominal wall. CT of the abdomen is far more sensitive (detects <0.5 mL of free air) and also identifies the site of perforation. Absence of free air on X-ray does not exclude perforation (small perforations, sealed perforations).
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.