What is the recommended evaluation and management for an adult with diarrhea‑predominant irritable bowel syndrome without alarm features?

Evaluation and Management of Diarrhea-Predominant IBS Without Alarm Features

Diagnosis: Make It Positive, Not by Exclusion

In adults under 45 years with typical IBS-D symptoms and no alarm features, establish the diagnosis confidently using symptom-based criteria plus limited baseline testing—extensive investigation is unnecessary and potentially harmful. 1, 2

The Rome criteria require recurrent abdominal pain at least 3 days per month for the past 3 months (with symptom onset ≥6 months prior) plus at least two of: pain relief with defecation, onset associated with change in stool frequency, or onset associated with change in stool form. 2 This symptom-based approach yields a positive likelihood ratio of 2–3 and negative likelihood ratio of 0.2–0.6. 1

Mandatory Baseline Testing (All Patients)

Perform this focused panel before making a final diagnosis: 2, 3

• Complete blood count (CBC) – to exclude anemia and inflammatory changes 2
• IgA tissue transglutaminase (IgA-tTG) with total IgA level – celiac disease mimics IBS-D with >90% sensitivity; if IgA-deficient, use IgG-deamidated gliadin peptide or IgG-tTG 2, 3
• Fecal calprotectin – values <50 µg/g exclude IBD with 97% specificity; >250 µg/g mandate colonoscopy 2, 3
• Stool testing for Giardia antigen – common parasitic cause of chronic diarrhea 2, 3
• Fecal occult blood test – screens for occult GI bleeding 2

Do NOT routinely order: CRP or ESR (poor diagnostic accuracy for IBD screening), ova and parasite testing beyond Giardia (unless travel to endemic areas), or serologic IBS antibody tests (anti-CdtB, anti-vinculin have <50% sensitivity and cannot rule out IBS-D). 1, 2, 3

Alarm Features That Mandate Extended Workup

Proceed directly to colonoscopy if ANY of the following are present: 1, 2, 3

Alarm Feature	Action Required
Age ≥45–50 years at symptom onset	Colonoscopy mandatory
Unintentional weight loss	Full diagnostic evaluation
Rectal bleeding or blood in stool	Endoscopic evaluation
Anemia on CBC	Investigate for bleeding/malabsorption
Nocturnal pain or diarrhea awakening from sleep	Extended workup (suggests organic disease)
Fever	Rule out diverticulitis, IBD
Family history of IBD or colorectal cancer	Lower threshold for colonoscopy

Critical caveat: Normal CRP does NOT exclude Crohn's disease—approximately 20% of active Crohn's patients have normal inflammatory markers. 2

When to Consider Additional Testing

For IBS-D Patients Who Fail Initial Therapy:

• Bile acid diarrhea testing – SeHCAT scanning (if available) or serum 7α-hydroxy-4-cholesten-3-one; 25–33% of suspected IBS-D patients have abnormal bile acid retention 1, 2, 3
• Lactose breath testing – only if consuming >280 mL (0.5 pint) milk daily, especially in high-risk ethnic groups 2, 3

For IBS-D With Specific Risk Factors for Microscopic Colitis:

Consider colonoscopy with biopsies (even from normal-appearing mucosa) if: female sex, age ≥50 years, coexistent autoimmune disease, nocturnal or severe watery diarrhea, symptom duration <12 months, weight loss, or use of NSAIDs/PPIs/SSRIs/statins. 1, 4 Microscopic colitis prevalence is approximately 1.5% in IBS-D cohorts. 4

Initial Management Algorithm

Step 1: Patient Education and Reassurance

Communicate a positive diagnosis clearly—IBS-D is a chronic disorder with fluctuating symptoms triggered by stress, illness, and eating, but it does NOT increase cancer risk or mortality. 1 This therapeutic communication alone improves outcomes. 2

Step 2: Dietary and Lifestyle Modifications

• Identify and eliminate common triggers: wheat, milk, coffee, potatoes, corn, onion, beef, oats, cheese, white wine 2
• Adequate hydration, reduce caffeine and alcohol 5
• Increase soluble fiber intake (may improve symptoms, though individual response varies) 5, 6
• Consider low-FODMAP diet trial 2

Step 3: First-Line Pharmacologic Therapy

For diarrhea and urgency:

• Loperamide (over-the-counter) – improves stool frequency and rectal urgency, though mixed results for abdominal pain 5, 6

For abdominal pain:

• Antispasmodics (hyoscyamine, dicyclomine) – target pain directly 2, 6

For refractory symptoms:

• Bile acid sequestrants (if bile acid diarrhea suspected) 5, 7
• Tricyclic antidepressants (low-dose) – for pain and comorbid psychological symptoms 5, 7, 6

Step 4: FDA-Approved Prescription Options for IBS-D

If first-line therapies fail after 3–6 weeks: 5, 7, 8

• Rifaximin – non-absorbable antibiotic, reduces global IBS-D symptoms 5, 7, 8
• Eluxadoline – mixed opioid receptor modulator, improves both pain and stool consistency 5, 7, 8
• Alosetron – 5-HT3 antagonist (restricted use due to ischemic colitis risk; reserved for severe cases in women) 5, 7

Avoid opioid analgesics – contraindicated in functional bowel disorders. 2

Follow-Up and Escalation

• Review at 4–6 weeks to assess response to dietary and pharmacologic interventions 2
• Refer to gastroenterology if: symptoms persist despite optimized therapy, new alarm features develop, atypical or severe symptoms emerge, or patient reaches age ≥45–50 years with ongoing symptoms 2, 4

Common Pitfalls to Avoid

• Over-testing young patients (<45 years) without alarm features – colonoscopy in this group is not cost-effective and delays appropriate care 2, 4, 3
• Serial repetitive testing after functional diagnosis is established – increases anxiety without diagnostic yield 2
• Relying solely on patient-reported food intolerances without objective testing – leads to unnecessary dietary restrictions 1, 2
• Assuming normal inflammatory markers exclude IBD – 20% of active Crohn's disease presents with normal CRP 2
• Performing routine colonoscopy or gastroscopy in typical IBS-D without alarm features – no indication and does not improve outcomes 4