What is the likely diagnosis and appropriate work‑up and management for a 43‑year‑old man with a one‑year history of crampy lower‑abdominal pain, alternating constipation and diarrhea, normal laboratory studies, and mild diverticulosis on imaging?

Likely Diagnosis: Irritable Bowel Syndrome (IBS)

This 43-year-old man with one year of crampy lower abdominal pain, alternating constipation and diarrhea, normal laboratory studies, and only mild diverticulosis on imaging most likely has irritable bowel syndrome (IBS), not symptomatic diverticular disease. 1, 2

Why IBS is the Primary Diagnosis

• Age and symptom pattern favor IBS: At 43 years, this patient fits the typical IBS demographic with chronic symptoms (>1 year), crampy abdominal pain, and alternating bowel habits—all classic Rome criteria features. 1, 2

• Mild diverticulosis is likely incidental: Only 5-10% of people have diverticulosis by age 45, and of those with diverticulosis, only 1-4% develop acute diverticulitis in their lifetime. 1, 3 The presence of mild diverticulosis does not explain chronic symptoms lasting one year. 4

• Absence of diverticulitis features: Acute diverticulitis presents with the clinical triad of left lower quadrant pain, fever, and leukocytosis—none of which this patient has. 1 His normal blood tests and lack of fever effectively exclude active diverticulitis. 1

• Symptomatic uncomplicated diverticular disease (SUDD) versus IBS: While SUDD can mimic IBS with recurrent abdominal pain and altered bowel habits, SUDD patients are typically older, lack female predominance, and often have fever with prolonged pain episodes—features absent in this case. 4

Essential Diagnostic Work-Up

Baseline Screening Tests (All Patients)

• Complete blood count (CBC): Already done and normal—this excludes anemia and inflammatory changes. 2, 5

• C-reactive protein (CRP) or ESR: Should be performed to exclude inflammatory bowel disease, though normal values do not completely rule out IBD (20% of active Crohn's disease patients have normal CRP). 1, 2, 5

• Celiac disease serology: IgA tissue transglutaminase (IgA-tTG) with total IgA level is mandatory, as celiac disease commonly presents with IBS-like symptoms. 2 If IgA deficient, use IgG-based testing (IgG-deamidated gliadin peptide). 2

• Fecal calprotectin: Values <50 μg/g effectively exclude inflammatory bowel disease; values >200-250 μg/g suggest IBD. 2, 5 This is particularly useful in patients under 45 with diarrhea. 2

• Stool testing for Giardia: This parasitic infection is a common cause of chronic diarrhea and IBS-like symptoms. 2

• Fecal occult blood test: Recommended as a screening measure for occult gastrointestinal bleeding. 2

Tests NOT Needed in This Case

• Colonoscopy is NOT indicated: In patients <45 years with typical IBS symptoms and no alarm features (no weight loss, no rectal bleeding, no nocturnal symptoms, no anemia), colonoscopy is not cost-effective and should not be performed. 1, 2, 5 The CT scan has already excluded structural pathology.

• Repeat CT imaging is unnecessary: The patient already has adequate imaging showing only mild diverticulosis. 1

• Hydrogen breath testing for SIBO: Not recommended in patients with typical IBS symptoms. 2

• Ultrasound: Not recommended as it often detects incidental findings unrelated to symptoms. 2

Alarm Features That Would Change Management

This patient has none of the following alarm features, which is reassuring:

Alarm Feature	Why It Matters
Age ≥45 years at new symptom onset	Would mandate colonoscopy to exclude colorectal cancer [2,5]
Rectal bleeding or blood in stool	Requires endoscopic evaluation [2,5]
Unintentional weight loss	Suggests malignancy or inflammatory disease [2,6]
Anemia on CBC	Excludes functional IBS; indicates organic disease [2,5]
Fever	Suggests systemic inflammation or infection [1]
Nocturnal symptoms (pain/diarrhea waking patient from sleep)	Specifically excludes IBS and indicates organic pathology [2,6]
Family history of IBD or colorectal cancer	Increases pre-test probability of serious disease [2]

Management Algorithm

Step 1: Confirm IBS Diagnosis Using Rome Criteria

• Rome criteria require: Abdominal pain for ≥12 weeks in the past 12 months with at least two of the following: 1, 2

  • Pain relieved by defecation ✓
  • Onset associated with change in stool frequency ✓ (alternating constipation/diarrhea)
  • Onset associated with change in stool form ✓

• This patient meets Rome criteria for IBS with alternating bowel habits. 1, 7

Step 2: Complete Baseline Laboratory Screening

• Order: CBC (done), CRP, celiac serology (IgA-tTG + total IgA), fecal calprotectin, stool for Giardia, fecal occult blood. 2

• If celiac serology is positive, small bowel biopsy during upper endoscopy is required for confirmation. 2

Step 3: Provide Firm Diagnosis and Reassurance

• Make a positive diagnosis of IBS based on symptom pattern and negative screening tests—do not present it as a "diagnosis of exclusion." 1, 2

• Explain that mild diverticulosis is an incidental finding present in 5-10% of people his age and does not cause his chronic symptoms. 1, 3

• Detailed explanation and reassurance are therapeutic and should be provided without immediately prescribing new medications. 1

Step 4: Initial Management

• Dietary modification: Consider lactose breath testing if he consumes >0.5 pint (280 mL) of milk daily, especially given variable lactose intolerance prevalence. 1, 2 A low-FODMAP diet may be beneficial. 6

• Fiber supplementation: High-fiber diet appears to decrease IBS symptoms, though the patient has already tried "my battery" (likely fiber) without much benefit. 8

• Avoid opiates: These should never be used for pain management in functional bowel disorders. 6

• Pharmacologic options: Based on predominant symptom (constipation vs. diarrhea), consider antispasmodics, laxatives, or antidiarrheals as appropriate. 1

Step 5: Follow-Up and Red Flags

• Schedule follow-up in 4-6 weeks to assess response to initial management. 1

• Immediate re-evaluation needed if: New onset of alarm features (bleeding, weight loss, fever, nocturnal symptoms, anemia). 2, 6

• Colonoscopy becomes indicated if: Patient turns 45 years old, develops alarm features, or symptoms significantly worsen despite appropriate IBS management. 2, 5

Common Pitfalls to Avoid

• Do not attribute chronic symptoms to mild diverticulosis: Diverticulosis is asymptomatic in the vast majority of cases; only 1-4% develop acute diverticulitis. 3 Chronic pain and altered bowel habits for one year are not explained by mild diverticulosis. 4, 8

• Do not perform colonoscopy in young patients with typical IBS and no alarm features: This is not cost-effective and exposes the patient to unnecessary risk. 1, 2

• Do not delay diagnosis waiting for "all tests to be normal": IBS is a positive diagnosis based on symptom criteria, not a diagnosis of exclusion. 1, 2

• Do not assume normal CRP excludes all inflammatory disease: Approximately 20% of patients with active Crohn's disease have normal CRP. 2, 5

• Do not skip celiac serology: This is mandatory screening in all patients with IBS-like symptoms, as celiac disease is a common mimic with sensitivity >90%. 2

• Do not initiate empiric treatment without completing basic screening: While extensive testing is unnecessary, the focused panel (CBC, celiac serology, fecal calprotectin, Giardia) should be completed to avoid missing treatable organic disease. 2