What is the diagnostic workup for Irritable Bowel Syndrome (IBS)?

Diagnostic Workup for Irritable Bowel Syndrome (IBS)

The diagnosis of IBS should be based on symptom-based criteria (Rome criteria) combined with limited diagnostic testing to exclude organic disease, rather than extensive investigations. 1

Symptom-Based Diagnosis

• IBS should be diagnosed using the Rome criteria, which define IBS as recurrent abdominal pain or discomfort for at least 12 weeks in the preceding 12 months associated with two or more of the following features 1:

  1. Relief with defecation
  2. Onset associated with a change in frequency of stool
  3. Onset associated with a change in form (appearance) of stool

• Supporting symptoms that strengthen the diagnosis include 1:

  • Abnormal stool frequency (>3 BM/day or <3 BM/week)
  • Abnormal stool form (lumpy/hard or loose/watery)
  • Abnormal stool passage (straining, urgency, feeling of incomplete evacuation)
  • Passage of mucus
  • Bloating or abdominal distention

• For primary care settings, the National Institute for Health and Care Excellence (NICE) definition may be more pragmatic: abdominal pain or discomfort with altered bowel habits for at least 6 months, in the absence of alarm symptoms 1

Initial Diagnostic Testing

• All patients presenting with IBS symptoms for the first time should have 1:

  • Full blood count
  • C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR)
  • Celiac disease serology (IgA-tTG is most sensitive and specific) 1
  • In patients <45 years with diarrhea, fecal calprotectin to exclude inflammatory bowel disease

• Stool studies should include 1:

  • Fecal occult blood test
  • Tests for ova and parasites (especially in areas with endemic infection)
  • Specific testing for Clostridioides difficile toxin when indicated

Alarm Features Requiring Further Investigation

• Presence of any of these features warrants additional testing 1:
  • Age ≥50 years at symptom onset
  • Unintentional weight loss
  • Rectal bleeding
  • Nocturnal symptoms that wake the patient
  • Family history of colorectal cancer, inflammatory bowel disease, or celiac disease
  • Anemia
  • Palpable abdominal or rectal mass
  • Fever

Additional Testing Based on IBS Subtype

For IBS with Diarrhea (IBS-D)

• Consider the following when diarrhea is the predominant symptom 1:
  • Lactose/dextrose hydrogen breath test if lactose intolerance is suspected
  • 23-seleno-25-homotaurocholic acid (SeHCAT) scan or serum 7α-hydroxy-4-cholesten-3-one to exclude bile acid diarrhea, particularly in patients with nocturnal diarrhea or prior cholecystectomy
  • Colonoscopy with biopsies to exclude microscopic colitis in patients with risk factors (female sex, age ≥50 years, coexistent autoimmune disease, nocturnal or severe watery diarrhea, duration <12 months, weight loss, or use of NSAIDs/PPIs/SSRIs/statins) 1

For IBS with Constipation (IBS-C)

• Consider the following when constipation is predominant 1:
  • Trial of fiber supplementation initially
  • If symptoms persist, evaluate for slow transit with whole gut transit test
  • Consider anorectal manometry or defecography if obstructed defecation is suspected

For IBS with Pain as Predominant Symptom

• Consider plain abdominal radiography during an acute pain episode to exclude bowel obstruction 1

When to Refer to Gastroenterology

• Referral to gastroenterology is warranted in the following circumstances 1:
  • Diagnostic uncertainty
  • Symptoms that are severe or refractory to first-line treatments
  • Presence of alarm features
  • Patient request for specialist opinion
  • Age >45 years with new-onset symptoms

Common Pitfalls to Avoid

• Avoid treating IBS as a diagnosis of exclusion requiring extensive testing 2, 3
• Recognize that colonoscopy has a very low yield in typical IBS without alarm features 1
• Be aware that 80% of IBS patients may report at least one alarm symptom, so clinical judgment is essential 1
• Avoid repeated diagnostic testing in patients with established IBS who have no new symptoms 1
• Remember that small intestinal bacterial overgrowth testing is not recommended as part of routine IBS workup 1

Follow-up Evaluation

• After initial diagnosis and treatment, reevaluate the patient in 3-6 weeks 1
• If treatment is unsuccessful or new symptoms develop, consider additional testing based on predominant symptoms 1
• Recognize that IBS is a chronic condition with fluctuating symptoms that may require ongoing management 1

By following this diagnostic approach, clinicians can make a positive diagnosis of IBS while excluding organic disease in a cost-effective manner, leading to improved patient outcomes and quality of life 4, 5.