What are the symptoms, labs, and imaging studies used to diagnose and rule out irritable bowel disease (IBD) in a patient presenting with abdominal pain, diarrhea, and weight loss?

Inflammatory Bowel Disease: Signs, Symptoms, and Diagnostic Workup

Clinical Presentation

IBD typically presents with abdominal pain, diarrhea, and weight loss, often accompanied by systemic symptoms including fever, malaise, and anorexia—features that distinguish it from functional bowel disorders. 1

Key Symptoms to Identify

• Ulcerative colitis presents with bloody diarrhea, urgency, rectal bleeding, and abdominal pain, with symptoms typically confined to the colon 1
• Crohn's disease manifests more heterogeneously with abdominal pain, diarrhea, weight loss, and may cause intestinal obstruction, fistulae (especially perianal), or abscesses 1
• Systemic symptoms (fever, malaise, anorexia) are more common in Crohn's disease than ulcerative colitis 1
• Extraintestinal manifestations affecting joints, skin, and eyes should be specifically queried 1

Critical Red Flag Features

• Fever, unintentional weight loss, blood in stools, anemia, and abnormal physical findings mandate immediate investigation 1
• Nocturnal diarrhea is a particularly important alarm feature 2
• Recent onset of symptoms in patients over 50 years requires aggressive workup 3
• Family history of IBD or colorectal cancer elevates pretest probability 1

Essential Laboratory Testing

For screening purposes, obtain CBC, ESR or CRP, stool Hemoccult, and fecal calprotectin—this combination provides the most efficient initial assessment. 1, 4

First-Line Blood Tests

• Complete blood count to assess for anemia and inflammatory changes 1, 5
• ESR or CRP to detect systemic inflammation, though 15-20% of patients with active Crohn's disease may have normal CRP 4, 5
• Serum chemistries and albumin to assess nutritional status and disease severity 1
• Celiac serology (IgA tissue transglutaminase with total IgA) to exclude celiac disease, which can mimic IBD 5, 6

Stool Studies

• Fecal calprotectin is the single most important stool test, with 93-95% sensitivity and 91-96% specificity for differentiating IBD from non-IBD diagnoses 4
• Values <50 μg/g effectively exclude IBD and point toward functional disorders 4
• Values >200-250 μg/g strongly suggest IBD and mandate endoscopic evaluation 4
• Intermediate values (50-250 μg/g) require clinical correlation and may warrant serial monitoring 4
• Stool culture and Clostridioides difficile toxin testing are mandatory in all new presentations of diarrhea 4, 5
• Stool for ova and parasites should be ordered based on geographic area and travel history 1
• Fecal occult blood testing for screening purposes 1

Important Pitfall to Avoid

Never rely solely on CRP to exclude IBD—up to 20% of patients with active Crohn's disease have normal CRP levels, and therapeutic decisions should never be based on clinical considerations alone without objective inflammatory markers. 4, 5

Imaging and Endoscopic Evaluation

Initial Imaging

• Plain abdominal radiography is essential in the initial assessment to exclude colonic dilatation, assess disease extent in ulcerative colitis, identify masses in Crohn's disease, or detect small bowel dilatation 1

Endoscopic Approach

• Rigid or flexible sigmoidoscopy should be performed in all patients presenting with diarrhea unless there are immediate plans for colonoscopy 1
• Macroscopic features of ulcerative colitis include loss of vascular pattern, granularity, friability, and ulceration of rectal mucosa 1
• Rectal biopsy should be obtained even if mucosa appears normal 1
• For mild-to-moderate disease, colonoscopy to the terminal ileum is preferable to assess disease extent and obtain biopsies 1
• For moderate-to-severe disease, flexible sigmoidoscopy is safer due to higher perforation risk with full colonoscopy 1
• Terminal ileal biopsy at colonoscopy may reveal microscopic evidence of Crohn's disease even when mucosa appears normal 1

Advanced Imaging

• Small bowel barium studies or cross-sectional imaging (CT/MRI enterography) are appropriate for suspected Crohn's disease to define extent and location 1
• Double-contrast barium enema may be used when colonoscopy is incomplete or contraindicated 1

Diagnostic Algorithm

The diagnosis of IBD is confirmed by clinical evaluation combined with biochemical, endoscopic, radiological, and histological findings—no single test is sufficient. 1

Step 1: History and Physical Examination

• Document stool frequency/consistency, urgency, rectal bleeding, abdominal pain, systemic symptoms, recent travel, medications, smoking status, and family history 1
• Examine for general wellbeing, vital signs, anemia, fluid depletion, weight loss, abdominal tenderness/masses, and perform perineal examination 1

Step 2: Initial Laboratory and Stool Testing

• CBC, ESR or CRP, serum chemistries, liver function tests 1
• Fecal calprotectin (>200-250 μg/g warrants endoscopy) 4
• Stool culture, C. difficile toxin, ova and parasites (if indicated) 1, 4
• Celiac serology 5, 6

Step 3: Imaging and Endoscopy Based on Results

• If fecal calprotectin >200-250 μg/g or alarm features present: proceed to colonoscopy with biopsies from affected and normal-appearing areas 4
• If plain radiography shows concerning features: proceed to appropriate cross-sectional imaging 1
• For suspected Crohn's disease: colonoscopy to terminal ileum plus small bowel imaging 1

Step 4: Histological Confirmation

• Ulcerative colitis diagnosis requires typical histological findings on biopsy with negative stool examinations for infectious agents 1
• Crohn's disease diagnosis depends on demonstrating focal, asymmetric, and often granulomatous inflammation 1

Critical Distinctions from Irritable Bowel Syndrome

IBS diagnosis requires absence of alarm features (fever, weight loss, blood in stools, anemia) and normal inflammatory markers—if any are present, pursue IBD workup. 1, 2

• IBS symptoms are relieved with defecation, associated with change in stool frequency or form, and include bloating without systemic symptoms 1
• IBS patients under age 45 without alarm features do not require colonoscopy 5
• Fecal calprotectin <50 μg/g in patients under 45 with diarrhea specifically excludes IBD and supports IBS diagnosis 4, 5