In an 18‑year‑old with acute hematochezia, abdominal cramping, recent constipation, difficulty gaining weight and no family history of gastrointestinal disease, what blood tests should be ordered to rule out Crohn’s disease and evaluate for irritable bowel syndrome?

Blood Tests for Evaluating Crohn's Disease and IBS in an 18-Year-Old with Hematochezia

In this 18-year-old presenting with acute hematochezia, abdominal cramping, constipation, and difficulty gaining weight, you should order: complete blood count (CBC), fecal calprotectin, IgA tissue transglutaminase with total IgA level, stool testing for Giardia, and fecal occult blood test—but the presence of hematochezia and weight loss are alarm features that mandate colonoscopy regardless of laboratory results. 1, 2, 3

Critical Context: This is NOT Typical IBS

The presence of hematochezia and difficulty gaining weight are absolute alarm features that exclude a simple functional IBS diagnosis and require endoscopic evaluation. 1, 3 While blood tests are important, they serve primarily to characterize the disease severity and exclude other conditions—colonoscopy with biopsies is mandatory in this clinical scenario. 1, 2

Essential Blood Tests to Order

Mandatory Initial Panel

• Complete Blood Count (CBC) – Screen for anemia (particularly iron deficiency from chronic blood loss) and inflammatory changes (elevated white blood cell count suggesting IBD). 1, 2, 3 Anemia has very high specificity for organic disease even though sensitivity is limited. 4

• Fecal Calprotectin – This is the single best non-invasive test to distinguish inflammatory bowel disease from IBS, with 100% sensitivity and 97% specificity when using a cutoff of >50 mg/g. 5, 6 In patients under age 45 with diarrhea or hematochezia, this test is particularly valuable. 1, 2 Fecal calprotectin correlates directly with intestinal inflammation and neutrophil activity. 7, 6

• IgA Tissue Transglutaminase (IgA-tTG) with Total IgA Level – Celiac disease must be excluded in all patients with chronic GI symptoms, as it has >90% sensitivity and can present with IBS-like symptoms. 1, 2 If IgA deficiency is detected, switch to IgG-based testing (IgG-deamidated gliadin peptide or IgG-tTG). 2

• Stool Testing for Giardia Antigen – Giardia is a common parasitic cause of chronic diarrhea that mimics IBD and IBS. 1, 2

Additional Useful Tests in This Context

• Fecal Occult Blood Test (Hemoccult) – Although the patient has visible hematochezia, this serves as baseline documentation. 1, 3

• C-Reactive Protein (CRP) – While the AGA guidelines suggest against routine CRP for screening, it can help characterize disease activity if IBD is confirmed. 5 However, approximately 20% of patients with active Crohn's disease have normal CRP levels, so a normal result does not exclude IBD. 1 CRP >5 mg/L has been used to distinguish IBD from IBS in multiple studies. 5

Tests You Should NOT Order

• Do NOT order ESR or CRP alone as screening tests—they have poor diagnostic accuracy for distinguishing IBS from IBD. 2, 4

• Do NOT order serologic antibody tests for IBS (anti-CdtB, anti-vinculin)—these have only 20-40% sensitivity and are not useful for clinical practice. 2

• Do NOT order routine ova and parasite testing unless there is specific travel history to endemic areas. 2

• Do NOT order abdominal ultrasound—it frequently detects incidental findings unrelated to symptoms and does not change management. 1, 4

Why Colonoscopy is Mandatory Here

The combination of hematochezia and weight loss overrides any laboratory testing algorithm. 1, 3 These are absolute alarm features requiring direct visualization and biopsy. 1, 2 The colonoscopy should include:

• Biopsies from both inflamed and normal-appearing mucosa 1
• Multiple biopsies to evaluate for microscopic colitis if diarrhea is present 1
• Terminal ileum intubation if possible to assess for Crohn's disease 1

Clinical Algorithm Summary

1. Order the mandatory blood panel immediately: CBC, fecal calprotectin, celiac serology (IgA-tTG + total IgA), stool Giardia antigen, fecal occult blood 1, 2

2. Refer for urgent colonoscopy due to alarm features (hematochezia + weight loss) 1, 3

3. Do NOT delay endoscopy waiting for laboratory results—the alarm features mandate visualization regardless of blood test results 1, 2

4. If fecal calprotectin is elevated (>50 mg/g), this strongly supports IBD over functional disease 5, 6

5. If celiac serology is positive, small bowel biopsy during upper endoscopy confirms diagnosis 5, 1

Common Pitfalls to Avoid

• Do not diagnose IBS in the presence of alarm features—hematochezia and weight loss exclude functional IBS by definition. 1, 8

• Do not rely on normal CRP to exclude Crohn's disease—up to 20% of active Crohn's patients have normal inflammatory markers. 1

• Do not perform extensive serologic testing panels—stick to the evidence-based tests listed above. 2, 8

• Do not order colonoscopy "if labs are abnormal"—the alarm features present in this case mandate colonoscopy regardless of laboratory results. 1, 3