In a 40‑year‑old man with two weeks of generalized abdominal pain and CT evidence of mild colitis, what are the likely differential diagnoses and appropriate management plan?

Differential Diagnosis and Management of a 40-Year-Old Male with Generalized Abdominal Pain and CT-Confirmed Mild Colitis

In a 40-year-old man presenting with two weeks of generalized abdominal pain and CT evidence of mild colitis, the most critical first step is comprehensive stool testing for infectious etiologies—including Clostridioides difficile toxin, bacterial cultures (Salmonella, Shigella, Campylobacter, E. coli), and ova/parasites—while simultaneously measuring fecal calprotectin to stratify inflammatory severity and guide the need for endoscopic evaluation. 1, 2

Differential Diagnosis

The CT finding of "mild colitis" in a 40-year-old male with two weeks of symptoms generates a broad differential that must be systematically narrowed:

Infectious Colitis (Most Common in Acute Presentations)

• Bacterial pathogens causing inflammatory diarrhea include Salmonella, Shigella, Campylobacter jejuni, enterohemorrhagic and enteroinvasive E. coli, Yersinia enterocolitica, and Clostridium difficile 3, 2, 4
• These typically present with bloody diarrhea, fever, tenesmus, and severe abdominal pain 3
• C. difficile must be tested regardless of antibiotic exposure history, as it is mandatory in all diarrheal presentations 1, 5
• The two-week duration makes bacterial gastroenteritis less likely but does not exclude it 2

Inflammatory Bowel Disease (IBD)

• Ulcerative colitis is a critical consideration given the patient's age (incidence peaks in early adulthood, with 132% increase in 40-49 year-olds from 1980-2007) 6, 7
• Presents with urgency, tenesmus, hematochezia, and abdominal pain 7, 8
• Left-sided colonic predominance on CT favors ulcerative colitis 1
• Fecal calprotectin >250 μg/g strongly correlates with active inflammation and ulceration 9

Diverticulitis

• Right or left colonic diverticulitis is increasingly common in this age group (incidence 180/100,000 per year in the U.S.) 6
• CT findings include colonic wall thickening and peri-colonic inflammatory changes 6
• Clinical suspicion alone is correct in only 40-65% of cases, necessitating CT confirmation 6

Ischemic Colitis

• Consider in patients with cardiovascular risk factors 5
• Left-sided colonic predominance is typical 1

Other Considerations

• Segmental colitis associated with diverticulosis (SCAD) can mimic IBD 6
• NSAID-induced colonic pathology if the patient uses NSAIDs 5
• Microscopic colitis requires histologic diagnosis 5

Immediate Diagnostic Workup

Laboratory Testing (Obtain Before Treatment)

• Complete blood count to assess for anemia and inflammatory changes 1, 5
• C-reactive protein and erythrocyte sedimentation rate (note: 15-20% of active IBD patients have normal CRP) 5, 7
• Serum albumin to assess nutritional status and severity 1, 5
• Electrolytes, urea/creatinine, and liver function tests 1

Stool Studies (Mandatory Before Initiating Therapy)

• Stool culture for Salmonella, Shigella, Campylobacter, and E. coli 1, 2
• C. difficile toxin assay (mandatory regardless of antibiotic history) 1, 5
• Ova and parasites if travel history or endemic exposure 1, 2
• Fecal calprotectin or lactoferrin: Values <50 μg/g effectively exclude IBD; >250 μg/g strongly suggest IBD but require endoscopic confirmation; intermediate values (50-250 μg/g) necessitate colonoscopy 1, 5

Endoscopic Evaluation

• Flexible sigmoidoscopy is preferred over full colonoscopy in acute presentations because it visualizes ~95% of inflammatory changes with lower perforation risk 1
• Indications for endoscopy include: elevated fecal inflammatory markers, persistent symptoms despite initial therapy, or unclear diagnosis 1
• Obtain biopsies from both affected and normal-appearing areas to detect microscopic colitis and differentiate IBD from infectious colitis 1, 5
• Full colonoscopy should be avoided if fulminant colitis or toxic megacolon is suspected 1

Management Algorithm

Step 1: Rule Out Infectious Causes

• Do not delay stool testing while awaiting endoscopy 1
• If C. difficile is suspected based on severity, initiate empiric oral vancomycin 125 mg four times daily while awaiting results 1
• For suspected bacterial colitis with fever and dysentery, consider empiric azithromycin 1000 mg single dose (covers Shigella, Salmonella, Campylobacter) 2
• Avoid empiric antibiotics for Shiga toxin-producing E. coli (STEC) if suspected (low-grade or no fever with acute dysentery) 2

Step 2: Supportive Care (All Patients)

• Intravenous fluids and electrolyte replacement (potassium, magnesium) 1
• Maintain adequate hydration to prevent nephrolithiasis (mesalamine-containing stones are undetectable by standard imaging) 10
• Avoid NSAIDs as they exacerbate colonic inflammation 1
• Avoid antiperistaltic agents and opiates to prevent toxic megacolon 1

Step 3: Severity Assessment and Treatment Escalation

If Infectious Colitis Confirmed:

• Pathogen-specific antimicrobial therapy for all organisms except STEC 2
• Discontinue unnecessary antibiotics promptly 1

If IBD Suspected (Elevated Calprotectin, Negative Infectious Workup):

• For mild to moderate ulcerative colitis: Initiate oral mesalamine 2.4-4.8 g once daily for induction, then 2.4 g daily for maintenance 10, 7
• Mesalamine should be taken with food, swallowed whole (not crushed), with adequate fluid intake 10
• Monitor renal function before and periodically during mesalamine therapy 10
• If symptoms worsen or fail to improve within 3-5 days, consider systemic corticosteroids or biologic therapy 1, 7

If Diverticulitis Confirmed:

• Uncomplicated diverticulitis: Outpatient management with oral antibiotics (though recent evidence questions routine antibiotic use) 6
• Complicated diverticulitis (abscess, peritonitis): Hospitalization, IV antibiotics, surgical consultation 6

Red Flags Requiring Urgent Intervention

• Severe abdominal pain with peritoneal signs (rebound tenderness, guarding, hypoactive bowel sounds) 1
• Fever >37.8°C, heart rate >90 bpm, hemoglobin <105 g/L, or CRP >30 mg/L (hospitalization criteria) 1
• Radiographic free air indicating perforation 1
• Colonic dilation >5.5 cm (toxic megacolon) 1
• Hemodynamic instability or septic shock 1
• Serum lactate >5.0 mmol/L (marker of severe disease warranting urgent colectomy) 1

Common Pitfalls to Avoid

• Do not rely on CT alone for definitive diagnosis—sensitivity for specific etiologies ranges only 53-85%, and imaging patterns overlap 1
• Do not assume normal CRP excludes active inflammation—15-20% of IBD patients have normal inflammatory markers 5
• Do not delay colonoscopy for serial stool testing in patients with persistent symptoms or elevated calprotectin 5
• Do not initiate immunosuppression before excluding C. difficile 9
• Do not use loperamide if infectious colitis is suspected 1