A 52-year-old man presents to the emergency department with acute onset severe epigastric pain, rigid abdomen, and signs of peritonitis. Clinical suspicion for perforated peptic ulcer is high. Which investigation is the gold standard for confirming pneumoperitoneum and guiding immediate surgical intervention?

Explanation

## Investigation of Choice for Pneumoperitoneum Detection **Key Point:** Erect chest X-ray (frontal view) is the gold standard first-line imaging for detecting free air in perforation because it provides maximum sensitivity for subdiaphragmatic air collection with minimal cost and radiation. ### Why Erect Chest X-ray is Optimal 1. **Subdiaphragmatic Air Collection**: Free air rises and collects under the diaphragm in upright patients, making it maximally visible on erect chest X-ray. 2. **High Sensitivity**: Detects as little as 1 mL of free air in optimal positioning. 3. **Rapid Bedside Availability**: Can be obtained within minutes in emergency settings without patient transport to CT suite. 4. **Cost-Effective**: Minimal radiation, no contrast required. ### Radiological Signs of Perforation on Erect CXR | Sign | Description | Sensitivity | | --- | --- | --- | | **Subdiaphragmatic air** | Lucent crescent under right or left hemidiaphragm | 60–80% | | **Pneumoperitoneum** | Air-fluid levels, lucency outlining bowel loops | Variable | | **Rigler's sign** | Visualization of both sides of bowel wall (air inside and outside) | Highly specific | **High-Yield:** Rigler's sign (visualization of both visceral and parietal peritoneal outlines of bowel wall) is pathognomonic for pneumoperitoneum but requires adequate free air volume. ### Role of Other Investigations - **Supine Abdominal X-ray**: Free air may not layer dependently; less sensitive than erect CXR. Used only if patient cannot sit upright. - **CT Abdomen/Pelvis**: Highly sensitive (>95%) and shows perforation site, but delays urgent surgical intervention; reserved for hemodynamically stable patients with diagnostic uncertainty. - **Abdominal Ultrasound**: Limited sensitivity for pneumoperitoneum; operator-dependent; not recommended as first-line. **Clinical Pearl:** In a clinically unstable patient with signs of peritonitis and high suspicion for perforation, a positive erect CXR is sufficient to proceed to emergency surgery without waiting for CT confirmation. **Tip:** If erect CXR is negative but clinical suspicion remains high (especially in supine/bedridden patients), obtain a left lateral decubitus view or proceed to CT for better sensitivity.