What is the appropriate work‑up and first‑line management for a patient with suspected diarrhea‑predominant irritable bowel syndrome?

IBS-Diarrhea Work-Up

For patients under 45 years with typical IBS-D symptoms and no alarm features, perform limited baseline testing only—full blood count, C-reactive protein or ESR, celiac serology, and fecal calprotectin—then make a confident positive diagnosis without colonoscopy. 1, 2

Essential Baseline Investigations

Perform these screening tests in all patients with suspected IBS-D:

• Full blood count to exclude anemia 1, 2, 3
• C-reactive protein or ESR to screen for inflammatory conditions 1, 2, 3
• Celiac serology (tissue transglutaminase antibodies) in all patients regardless of stool pattern, as celiac disease commonly mimics IBS-D 1, 2, 4
• Fecal calprotectin if age <45 years with diarrhea to exclude inflammatory bowel disease 1, 2, 5

When Colonoscopy IS Required

Proceed directly to colonoscopy with biopsies if any of these features are present:

• Age ≥45 years at symptom onset 1, 2, 6
• Alarm features: unintentional weight loss (≥5% in 6 months), rectal bleeding, family history of colorectal cancer or IBD, nocturnal symptoms that wake the patient from sleep 1, 7, 6
• Risk factors for microscopic colitis: female sex, age ≥50 years, coexistent autoimmune disease, nocturnal or severe watery diarrhea, duration <12 months, or use of NSAIDs, PPIs, SSRIs, or statins 1, 2, 6
• Positive fecal calprotectin to exclude IBD 5

Critical pitfall: Up to 80% of IBS patients report at least one alarm symptom, so clinical judgment is essential—weight loss and rectal bleeding have higher specificity for organic disease than other alarm features. 1, 7

When Colonoscopy Is NOT Needed

• Typical symptoms in patients <45 years without alarm features have extremely low yield on colonoscopy 1, 6
• Normal baseline investigations from primary care with typical symptoms make IBS diagnosis secure 1
• Colonoscopy does not provide reassurance to patients and should not be performed for this purpose 1, 6

Making the Positive Diagnosis

Once organic disease is excluded, communicate a confident positive diagnosis:

• Explain IBS-D as a disorder of gut-brain interaction with visceral hypersensitivity as the primary mechanism, not a psychological disorder 1, 2
• Emphasize benign prognosis: IBS does not increase cancer risk or mortality, though it significantly impacts quality of life similar to IBD 1
• Set realistic expectations: cure is unlikely, but substantial symptom improvement and quality of life enhancement are achievable with long-term treatment 1, 2
• Avoid fragmented care: do not refer to multiple specialists for each new symptom, as IBS patients commonly have multi-system complaints 1

Additional Testing in Select Cases

Consider these only when clinically indicated based on specific features:

• SeHCAT scan or empiric bile acid sequestrant trial if severe watery diarrhea persists despite standard treatment, as up to one-third of presumed IBS-D patients have bile acid diarrhea 5, 8
• Repeat colonoscopy only if new alarm features develop or symptoms dramatically change character 1

Do NOT perform: routine hydrogen breath testing for lactose malabsorption or small intestinal bacterial overgrowth, as these have no established role in IBS-D diagnosis. 5

Psychological Comorbidity Assessment

• Screen early for anxiety, depression, and somatization as these predict treatment response and guide therapy selection 1, 2
• Identify psychological comorbidities early to enable timely referral for gut-directed CBT or hypnotherapy, rather than waiting for multiple drug failures 1, 2, 6