A 68-year-old man presents to the emergency department with acute onset severe abdominal pain, abdominal distension, and vomiting. On examination, he has a rigid abdomen with rebound tenderness. Plain abdominal radiograph shows pneumoperitoneum. Which is the most appropriate next investigation to identify the site of perforation?

## Investigation of Choice for Locating Perforation Site **Key Point:** While plain radiograph confirms pneumoperitoneum, CT is the gold standard for identifying the site and cause of perforation and assessing for peritonitis and complications. ### Why CT Abdomen and Pelvis is Superior **High-Yield:** CECT (Contrast-Enhanced CT) abdomen and pelvis is the investigation of choice because it: - Identifies the site of perforation (duodenal, gastric, small bowel, colon) - Shows the cause (peptic ulcer, malignancy, diverticulitis, trauma) - Detects free fluid, abscess formation, and peritoneal thickening - Guides surgical planning and determines urgency - Has sensitivity >90% for detecting perforation site ### Plain Radiograph Limitations While upright chest radiograph or left lateral decubitus view can confirm pneumoperitoneum (free air under diaphragm), it does NOT: - Localize the perforation site - Show the underlying cause - Assess for complications (abscess, peritonitis) - Provide information for surgical approach ### Comparison Table: Investigations for Suspected Perforation | Investigation | Sensitivity | Specificity | Localizes Site | Shows Cause | Detects Complications | |---|---|---|---|---|---| | Plain radiograph (upright/lateral decubitus) | 60–80% | High | No | No | No | | Upright chest radiograph | 70–85% | High | No | No | No | | CECT abdomen/pelvis | >90% | >95% | Yes | Yes | Yes | | Ultrasound | 40–60% | Moderate | Poor | Poor | Moderate | | Barium meal | Contraindicated | — | Poor | Possible | No | **Clinical Pearl:** In acute perforation with peritonitis, barium studies are contraindicated due to risk of barium peritonitis and delay to surgery. **Warning:** Do not rely on plain radiograph alone for surgical decision-making; always proceed to CECT for definitive localization in a stable patient, or proceed directly to surgery if unstable. **Mnemonic: CECT for Perforation — Cause, Effect, Complication, Timing** - **C**ause of perforation (ulcer, malignancy, diverticulitis) - **E**xtent of free air and fluid - **C**omplications (abscess, peritonitis, sepsis) - **T**iming and urgency for surgical intervention [cite:Harrison 21e Ch 287]