Irritable Bowel Syndrome: Evaluation and Management
Red-Flag Symptom Screening
Before making an IBS diagnosis, actively exclude alarm features that mandate further investigation. Screen every patient for: new symptom onset after age 50, rectal bleeding (not from hemorrhoids/fissures), unintentional weight loss, iron deficiency anemia, nocturnal diarrhea, and family history of colon cancer or inflammatory bowel disease 1.
Initial Diagnostic Approach
Young patients (<45 years) with typical symptoms, no alarm features, and normal examination can receive a confident IBS diagnosis without extensive testing 2. Required screening tests are limited to:
- Full blood count
- C-reactive protein or ESR
- Coeliac serology (tissue transglutaminase antibodies) 3
Do NOT routinely test for fecal calprotectin or food allergies in uncomplicated cases 3.
For patients ≥45 years or those with alarm features, colonoscopy is warranted 2. Patients with diarrhea-predominant symptoms require additional evaluation: serum B12, red cell folate, ferritin, thyroid function, and stool microscopy 2.
Management Algorithm by IBS Subtype
IBS with Diarrhea (IBS-D)
First-line pharmacological options (in order of evidence strength):
Rifaximin - 550 mg three times daily for 14 days; can retreat if symptoms recur 1. Moderate-quality evidence supports both initial treatment and retreatment cycles.
Eluxadoline - Conditional recommendation with moderate certainty 1. Targets multiple mechanisms in IBS-D.
Alosetron - 0.5 mg twice daily initially, can increase to 1 mg twice daily after 4 weeks if needed 1. Restricted to women with severe IBS-D under risk management program due to ischemic colitis and constipation risks. Stop immediately if constipation develops.
Loperamide - 4-12 mg daily, either regularly or prophylactically before events 2. Very low-quality evidence for global IBS symptoms, but effective for stool frequency control as adjunctive therapy 1, 4.
Common pitfall: Alosetron must be discontinued if constipation occurs at any dose. Do not simply reduce the dose without first stopping completely 1.
IBS with Constipation (IBS-C)
Polyethylene glycol (PEG) laxatives are suggested for symptom relief, particularly for increasing bowel movement frequency 4. While evidence for global IBS symptom improvement is limited, the low cost, safety profile, and established efficacy for constipation support their use as adjunctive treatment.
All IBS Subtypes
Tricyclic antidepressants (TCAs) - Conditional recommendation with low-quality evidence 4. Provide modest improvement in global relief and abdominal pain. Start with low doses (e.g., amitriptyline 10-25 mg at bedtime). Caution in patients at risk for QT prolongation 4.
Antispasmodics - Conditional recommendation with low-quality evidence 4. Meta-analysis of 22 RCTs showed significant improvement in global symptoms and modest pain reduction. Use continuously, not as-needed, based on trial data 4.
SSRIs are NOT recommended - Conditional recommendation AGAINST use 4. Five RCTs showed no improvement in global symptoms or abdominal pain.
Dietary Management
Trial a low-FODMAP diet for 4-8 weeks 3. This is the most evidence-based dietary intervention. Do NOT recommend gluten-free diets unless celiac disease is confirmed 3.
Psyllium (soluble fiber) may reduce symptoms, but avoid wheat bran 3. For diarrhea-predominant patients, identify and reduce excessive intake of lactose (>280 ml milk/day), fructose, sorbitol, caffeine, or alcohol 2.
Peppermint oil is suggested as an alternative therapy 3.
Psychological Therapies
IBS-specific cognitive behavioral therapy (CBT) and gut-directed hypnotherapy are strongly recommended when symptoms persist after 12 months of drug treatment, though earlier referral is appropriate based on patient preference and local availability 5. Both have strong recommendations despite low-quality evidence, as they show durable benefit beyond the high placebo response seen with pharmacotherapy 2.
Follow-Up Strategy
Establish a positive diagnosis early and explain it clearly - Use simple analogies: "sensitive gut," "brain-gut interaction," stress-triggered symptoms 2. Address patient fears about cancer directly, as this is a primary concern 2.
Symptom diaries for 2 weeks help identify triggers and guide management discussions 2.
Reassess if symptoms progress or change character - This warrants re-evaluation for alternative diagnoses 2. The prognosis for remaining free of serious disease is excellent, though approximately 30% remain symptomatic at 5 years 2.
Critical Caveats
The placebo response in IBS trials averages 47%, approximately three times larger than additional drug effects (16%) 2. This underscores the importance of the therapeutic relationship and adequate explanation time.
Avoid opioids, unnecessary surgery, and unproven diagnostic approaches 5. These cause iatrogenic harm in severe/refractory cases.
For severe refractory IBS, review the diagnosis and consider combination neuromodulators (augmentation), but monitor for serotonin syndrome risk 5.