What diagnostic test will the surgeon recommend for a 56-year-old female with weight loss, history of constipation, and air under the diaphragm on abdominal X-ray?

CT Scan to Rule Out Colonic Cancer with Perforation

In a 56-year-old woman with weight loss, constipation, and free air under the diaphragm on plain X-ray, the surgeon should immediately order a CT scan with intravenous contrast to confirm the perforation, identify its location, and determine if colonic cancer is the underlying cause. 1

Why CT is the Definitive Next Step

The presence of pneumoperitoneum (air under the diaphragm) on plain radiography already confirms perforation with 92% positive predictive value 1. However, CT is essential for surgical planning because it:

• Identifies the exact site of perforation with 86-89% accuracy, which is critical for operative approach 1, 2
• Detects the underlying cause (cancer, diverticulitis, other pathology) that led to perforation 1, 2
• Demonstrates specific CT findings including focal wall defect/ulcer (84% of cases), wall thickening (72%), extraluminal gas (97%), and peritoneal fluid (89%) 1
• Provides staging information if cancer is present, assessing for metastases and extent of disease 1

Why the Other Options Are Inappropriate

Abdominal ultrasound (Option B) is inferior to CT for perforation diagnosis. While US performs better than plain X-ray, CT significantly outperforms both modalities for detecting perforation and identifying its cause 1. In emergency settings with suspected perforation, US lacks the sensitivity and specificity needed for surgical decision-making 1.

Colonoscopy (Option C) is absolutely contraindicated when perforation is suspected or confirmed. Insufflation of air during colonoscopy would worsen the pneumoperitoneum and contaminate the peritoneal cavity further 1. Colonoscopy is only appropriate in stable patients WITHOUT signs of perforation 1.

Barium enema (Option D) is strictly contraindicated in suspected perforation because barium causes severe chemical peritonitis if it leaks into the peritoneal cavity 1. Even water-soluble contrast enemas are contraindicated when perforation is already confirmed 1.

Clinical Context Supporting Malignancy

This patient's presentation strongly suggests colonic cancer with perforation:

• Weight loss is a red flag for malignancy 1
• Chronic constipation suggests progressive colonic obstruction from tumor 1
• Age 56 falls within the typical colorectal cancer demographic 1
• Perforation can occur either at the tumor site itself or proximal to an obstructing lesion (diastatic perforation) 1

The Surgical Algorithm

Once CT confirms perforation with suspected cancer:

1. Immediate resuscitation with IV fluids, broad-spectrum antibiotics, and NPO status 1
2. Urgent laparotomy for source control of sepsis—this is the priority over oncologic considerations 1
3. Oncologic resection should still be performed when feasible to achieve better long-term outcomes, with or without primary anastomosis depending on peritoneal contamination 1
4. Stoma creation is often necessary, though only a small proportion of patients undergo reversal 1

Critical Pitfall to Avoid

Do not delay CT imaging to pursue colonoscopy or other diagnostic tests. Free air under the diaphragm represents a surgical emergency requiring immediate confirmation of location and cause via CT, followed by prompt operative intervention 1. The diagnostic workup must be expedited—typically CT should be obtained within hours, not days 1.