In a patient with a fecal calprotectin of 2,500 µg/g and hematochezia, what additional work‑up is indicated and what are the differential diagnoses?

Elevated Fecal Calprotectin 2,500 µg/g with Hematochezia: Work-Up and Differential Diagnosis

A fecal calprotectin of 2,500 µg/g with blood in stool mandates urgent colonoscopy within 1–2 weeks, as this markedly elevated level indicates severe intestinal inflammation with >95% likelihood of moderate-to-severe endoscopic disease. 1, 2

Immediate Diagnostic Work-Up

Essential Laboratory Testing

• Complete blood count to assess for anemia (from chronic blood loss), leukocytosis (suggesting active inflammation or infection), and thrombocytosis (common in active IBD) 2, 3
• C-reactive protein and erythrocyte sedimentation rate to quantify systemic inflammation 1, 2
• Comprehensive metabolic panel to evaluate for dehydration, electrolyte abnormalities, and hepatic or renal dysfunction 2
• Celiac serology (tissue transglutaminase IgA with total IgA) if not previously performed, as celiac disease can present with similar symptoms 2

Mandatory Stool Studies

• Stool culture including C. difficile testing to exclude infectious colitis, particularly critical in patients on immunosuppressants or recent antibiotics 1, 2, 3
• Ova and parasite examination if travel history, immunosuppression, or epidemiologic risk factors are present 2
• Cytomegalovirus testing should be considered if severe disease or treatment-refractory symptoms develop 3

Endoscopic Evaluation

• Complete ileocolonoscopy with terminal ileum intubation is the gold standard, superior to flexible sigmoidoscopy alone 1, 2
• Multiple biopsies from all colonic segments and terminal ileum, including normal-appearing mucosa, are essential for histologic diagnosis and to differentiate UC from CD 1, 2
• At a calprotectin level of 2,500 µg/g, the probability of finding moderate-to-severe endoscopic inflammation approaches 95% in symptomatic patients 1, 2

Cross-Sectional Imaging Considerations

• MR enterography or CT enterography should be obtained if Crohn's disease is suspected based on clinical features (perianal disease, right lower quadrant pain, weight loss) to assess small bowel involvement and complications such as strictures or abscesses 1
• Imaging is not routinely required before colonoscopy unless obstructive symptoms, palpable mass, or concern for perforation are present 1

Differential Diagnosis

Primary Inflammatory Bowel Disease (Most Likely)

• Moderate-to-severe ulcerative colitis is the leading diagnosis given hematochezia and markedly elevated calprotectin; UC typically presents with bloody diarrhea, urgency, and tenesmus 1
• Crohn's disease with colonic involvement should be considered, particularly if there are features such as perianal disease, skip lesions on endoscopy, or transmural inflammation on imaging 1
• Indeterminate colitis may be diagnosed if histologic and endoscopic features do not clearly distinguish UC from CD 1

Infectious Colitis

• Bacterial enterocolitis (Salmonella, Campylobacter, Shigella, enterohemorrhagic E. coli) can produce calprotectin levels >1,000 µg/g and must be excluded before initiating immunosuppression 1, 4, 5
• C. difficile infection is critical to rule out, especially in patients with recent antibiotic exposure or healthcare contact 1, 3
• Cytomegalovirus colitis occurs in immunocompromised patients or those with severe UC refractory to treatment 3

Ischemic Colitis

• Ischemic colitis typically affects older patients with vascular risk factors and presents with sudden-onset abdominal pain and bloody diarrhea; calprotectin is elevated but usually not to this extreme degree 2, 5
• Colonoscopy reveals segmental involvement, often in watershed areas (splenic flexure, rectosigmoid junction) 2

Colorectal Neoplasia

• Colorectal cancer or advanced adenoma can elevate calprotectin, though levels >2,000 µg/g are uncommon unless there is superimposed inflammation 1, 6, 5
• Alarm features such as weight loss, iron-deficiency anemia, or change in bowel habit mandate cancer pathway referral regardless of calprotectin 1, 2
• Fecal calprotectin lacks sufficient sensitivity to exclude colorectal cancer; colonoscopy remains mandatory 1, 2

Other Inflammatory Conditions

• Microscopic colitis (lymphocytic or collagenous) typically presents with watery diarrhea without blood and lower calprotectin levels, making it less likely here 2
• Radiation colitis should be considered in patients with prior pelvic radiation 5
• NSAID-induced enterocolopathy can elevate calprotectin; recent NSAID use within 6 weeks warrants consideration 1, 2
• Helicobacter pylori gastritis can elevate calprotectin but typically to much lower levels (mean ~240 µg/g) and does not cause hematochezia 7

Critical Clinical Caveats

False Reassurance from Normal FIT

• A normal fecal immunochemical test (FIT) does not exclude IBD or other significant organic pathology, as FIT is optimized for colorectal cancer detection and has poor sensitivity for inflammatory conditions 2
• The combination of markedly elevated calprotectin with normal FIT actually increases the likelihood of IBD rather than malignancy 2

Infection Must Be Excluded Before Immunosuppression

• Stool cultures and C. difficile testing must be completed before initiating or escalating immunosuppressive therapy, as treating infection with immunosuppression can lead to fulminant colitis or toxic megacolon 1, 3
• Immunosuppressive medications should be discontinued or adjusted during active infection 3

Calprotectin Interpretation in Context

• Calprotectin >250 µg/g strongly suggests active disease requiring treatment intensification in confirmed IBD 1, 2
• At levels >150 µg/g with moderate-to-severe symptoms, the false-positive rate is only 4.6%, meaning 95.4% of patients have true moderate-to-severe endoscopic inflammation 1, 2
• Calprotectin is not specific for IBD; infectious gastroenteritis, colorectal cancer, NSAID use, and local bleeding from hemorrhoids can all elevate levels 1, 2, 5

Post-Endoscopy Management Framework

If IBD Is Confirmed

• Treatment intensity should match disease severity; calprotectin >250 µg/g typically requires immunomodulator or biologic therapy rather than aminosalicylates alone 2
• Repeat calprotectin at 2–4 months after treatment initiation to assess biochemical response, targeting <150 µg/g as an indicator of adequate therapeutic response 2
• Follow-up colonoscopy at 6–12 months is recommended to confirm mucosal healing, recognizing that calprotectin normalization correlates with but does not guarantee endoscopic remission 2
• Serial calprotectin monitoring every 3–6 months in established IBD patients can facilitate early detection of impending flares 2

If Infection Is Identified

• Treat the specific pathogen appropriately and reassess symptoms and calprotectin after treatment completion 3
• Repeat calprotectin 4–6 weeks after infection resolution to determine if persistent elevation suggests underlying IBD 1, 2