In a 54-year-old man with new left lower quadrant pain, constipation and watery stools after starting a high‑protein diet, what is the next step in management?

Next Step: Order CT Abdomen and Pelvis with IV Contrast Immediately

In a 54-year-old man with left lower quadrant pain, constipation, and watery stools after starting a high-protein diet, you should order a CT abdomen and pelvis with intravenous contrast as the next step. This is the gold-standard imaging modality rated 8/9 (usually appropriate) by the American College of Radiology, with 98% diagnostic accuracy for identifying diverticulitis and other life-threatening causes of left lower quadrant pain 1, 2.

Why CT with IV Contrast is Mandatory

Clinical examination alone is unreliable, with misdiagnosis rates of 34-68% for left lower quadrant pain 2, 3. The classic triad of left lower quadrant pain, fever, and leukocytosis occurs in only 25% of diverticulitis cases, making imaging essential even when fever is absent 2.

Key Benefits of CT with IV Contrast

• Confirms or excludes diverticulitis with near-perfect accuracy (sensitivity 95-99%, specificity 99-100%) 1, 3, 4
• Detects complications that determine whether the patient needs emergency surgery, percutaneous drainage, antibiotics, or can be managed as an outpatient 1, 2
• Identifies alternative diagnoses in approximately 49% of patients with non-localized pain, including perforated colon cancer, inflammatory bowel disease, bowel obstruction, epiploic appendagitis, and ischemic colitis 2, 5
• Reduces hospital admissions by >50% through accurate risk stratification 2

Critical Differential Diagnosis to Rule Out

Perforated Colon Cancer (Most Dangerous Missed Diagnosis)

Colon cancer can mimic both the clinical presentation and CT appearance of diverticulitis 3, 2. CT findings that favor cancer over diverticulitis include:

• Pericolonic lymphadenopathy >1 cm in short axis with or without pericolonic edema 3, 2
• Luminal mass on CT 2

In contrast, diverticulitis typically shows inflammatory changes and edema in the root of the sigmoid mesentery without pericolonic lymphadenopathy 3.

Other Important Differentials

• Inflammatory bowel disease: skip lesions or multi-segment bowel wall thickening 2
• Bowel obstruction: dilated proximal bowel loops, especially concerning given the constipation history 2
• Epiploic appendagitis: pericolic fatty mass with hyperattenuated rim 2
• Ischemic colitis: vascular distribution pattern 6

What to Look for on CT

The radiologist should specifically assess for:

• Bowel wall thickening with pericolonic fat stranding (hallmark of diverticulitis) 4, 2
• Abscess formation and size (determines drainage strategy: <4 cm = antibiotics alone; ≥4 cm = percutaneous drainage) 2
• Free air or extraluminal gas indicating perforation 4, 2
• Fistula formation (colovesical, colovaginal, coloenteric) 1
• Pericolonic lymphadenopathy suggesting malignancy rather than diverticulitis 3, 2

Management Algorithm Based on CT Results

Uncomplicated Diverticulitis (No Abscess, Perforation, or Fistula)

• Immunocompetent patients: Conservative management without antibiotics is recommended 2, 3
• Immunocompromised or elderly patients: Antibiotics for maximum 7 days (Ertapenem 1g q24h or Eravacycline 1mg/kg q12h) 2
• Outpatient management is appropriate if the patient can tolerate oral intake, has no peritoneal signs, and has reliable follow-up 2

Complicated Diverticulitis with Small Abscess (<4 cm)

• Antibiotics alone for 7 days without drainage 2

Complicated Diverticulitis with Large Abscess (≥4 cm)

• Percutaneous CT-guided drainage plus antibiotics for 4 days in hemodynamically stable patients 2

Perforation with Diffuse Peritonitis or Free Air

• Emergency surgical consultation for possible Hartmann's procedure or primary resection with anastomosis 2

Common Pitfalls to Avoid

Do not assume diverticulitis based solely on a history of diverticulosis or dietary changes 2. The recent high-protein diet may have triggered symptoms, but alternative pathologies—especially colon cancer—must be excluded with imaging 3.

Do not order plain abdominal radiographs as the initial test 2. They have extremely low sensitivity for diverticulitis and will delay definitive diagnosis 1.

Do not rely on ultrasound as the primary imaging modality 1, 2. While graded-compression ultrasound has reported sensitivity of 77-98%, it is highly operator-dependent and limited in obese patients 1.

Do not perform colonoscopy during the acute episode 1. Colonic distention increases the risk of perforation in acute diverticulitis 1. Colonoscopy should be delayed 6-8 weeks after symptom resolution if indicated 3.

When to Repeat Imaging

Order repeat CT abdomen/pelvis with IV contrast if:

• Symptoms persist for >2-3 days despite conservative management 2
• Fever and leukocytosis persist beyond 48-72 hours despite appropriate treatment 5
• Clinical deterioration occurs at any time 5