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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
Key points
- IBS = recurrent abdominal pain at least 1 day/week for 3 months associated with defecation or stool frequency/form change
- No alarm features and typical symptoms: limited testing, not exhaustive exclusion
- IBS-D should prompt celiac serology and inflammatory marker/fecal calprotectin consideration
- Alarm features include GI bleeding, weight loss, anemia, nocturnal diarrhea, fever, family history, or onset after age 50
- Treatment is subtype-specific: fiber/PEG for IBS-C, loperamide/rifaximin for IBS-D, neuromodulators for pain
Overview
IBS is a disorder of gut-brain interaction with visceral hypersensitivity, altered motility, microbiome changes, and psychosocial modulators. It does not cause bleeding, anemia, fever, weight loss, or objective inflammation.
Epidemiology
IBS is common in primary care and gastroenterology, with higher prevalence in women, patients with anxiety/depression, and after infectious gastroenteritis. Subtypes include IBS-D, IBS-C, mixed IBS, and unclassified IBS.
Clinical Features
Symptoms
Recurrent abdominal pain associated with defecation
Change in stool frequency or form
Bloating and visible distension
Diarrhea, constipation, or alternating pattern
Rectal bleeding, weight loss, nocturnal diarrhea, fever, or anemia
Signs
Normal exam or mild diffuse tenderness
No mass or objective systemic illness
Pallor, abdominal mass, fever, or focal peritonitis
Investigations
First-line
Rome IV criteriaRecurrent abdominal pain at least 1 day/week in last 3 months associated with defecation or stool change
CBCScreen if anemia or alternative disease suspected
Second-line
Celiac serologytTG-IgA plus total IgA in IBS-D or mixed symptoms
CRP or fecal calprotectinLow values make IBD unlikely
Stool testingOnly if infection risk or persistent severe diarrhea
Specialist
ColonoscopyOnly age-appropriate screening or alarm features
1
Core management
- Positive diagnosis and reassurance
- Low-FODMAP trial with reintroduction
- Soluble fiber such as psyllium; avoid chronic opioids
2
IBS-D
- Loperamide for stool frequency
- Rifaximin for selected IBS-D global symptoms
- Eluxadoline only if no contraindications
3
IBS-C/pain
- PEG for constipation frequency
- Linaclotide, plecanatide, or lubiprostone in selected patients
- Antispasmodics, TCAs, CBT, or gut-directed hypnotherapy for pain
Complications
- Quality-of-life impairment: Work absence and dietary restriction.
- Medication harms: Opioids worsen bowel dysfunction.
- Missed organic disease: Alarm features require evaluation.
USMLE Step 2 CK Exam Tips
- 1IBS requires abdominal pain; diarrhea without pain is not IBS
- 2No blood, anemia, fever, nocturnal diarrhea, or weight loss in uncomplicated IBS
- 3IBS-D: test for celiac disease
- 4Fecal calprotectin helps separate IBS from IBD
- 5Post-infectious IBS follows gastroenteritis
- 6Loperamide helps diarrhea but not global pain
- 7New IBS after age 50 or with iron deficiency anemia = colonoscopy
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regional clinical guidance
Irritable Bowel Syndrome: guidance by region
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