In a 16‑year‑old male with five months of abdominal pain and bloating, a negative infectious diarrhea panel, markedly elevated fecal calprotectin, elevated C‑reactive protein, and negative tissue transglutaminase immunoglobulin A antibody, what is the most likely diagnosis?

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Inflammatory Bowel Disease (Crohn's Disease)

The most likely diagnosis in this 16-year-old male is inflammatory bowel disease, specifically Crohn's disease, given the markedly elevated fecal calprotectin (3177 μg/g) and elevated CRP (49 mg/L) in the context of chronic gastrointestinal symptoms.

Diagnostic Reasoning

Why IBD is the Primary Diagnosis

The combination of findings strongly points toward active intestinal inflammation:

  • Fecal calprotectin of 3177 μg/g is dramatically elevated - This is more than 10-fold higher than the threshold of 250 μg/g that predicts endoscopically active IBD with 76% sensitivity and 74% specificity 1. In pediatric studies, fecal calprotectin above 744 μg/g predicts IBD with 86.8% specificity and 66.7% sensitivity 2. Your patient's value far exceeds even this threshold.

  • CRP of 49 mg/L indicates systemic inflammation - While CRP is less specific than fecal calprotectin for intestinal inflammation 3, an elevated CRP above 5 mg/L has 67% sensitivity and 73% specificity for endoscopically active disease 1. A value of 49 mg/L strongly suggests active inflammation.

  • Negative tissue transglutaminase IgA effectively rules out celiac disease - This is the appropriate screening test for celiac disease 4, and the negative result (<0.5) excludes this diagnosis.

  • Five-month duration with negative infectious workup - The chronicity and negative infectious panel make infectious causes unlikely 4.

Crohn's Disease vs. Ulcerative Colitis

At age 16, this patient falls into the Montreal classification A1 category (diagnosis ≤16 years) 3. While both Crohn's disease and ulcerative colitis are possible, the clinical presentation with abdominal pain and bloating (rather than primarily bloody diarrhea) slightly favors Crohn's disease, though definitive differentiation requires endoscopic and histologic evaluation.

Immediate Next Steps

This patient requires urgent gastroenterology referral and endoscopic evaluation (both upper endoscopy and colonoscopy with biopsies) to:

  1. Confirm the diagnosis of IBD - Endoscopy with histology is the gold standard 3, 1
  2. Classify disease location and behavior - Essential for treatment planning using Montreal classification 3
  3. Assess disease severity - To guide initial therapy decisions
  4. Rule out microscopic colitis - Though less likely given the extremely elevated biomarkers, random biopsies during colonoscopy are recommended 4

Important Caveats

  • Do not delay endoscopy based on these biomarker levels - In symptomatic patients with moderate to severe symptoms and elevated biomarkers, endoscopic assessment should proceed without delay 1. The extremely elevated fecal calprotectin (>3000 μg/g) and CRP (49 mg/L) in a symptomatic patient warrant immediate evaluation rather than empiric treatment.

  • Consider additional workup - Check complete blood count (for anemia, which is common in IBD), albumin (for nutritional status), and stool studies if not already comprehensive (to definitively exclude infectious causes including C. difficile).

  • Avoid empiric corticosteroid therapy before endoscopy - This could mask endoscopic findings and delay definitive diagnosis.

Why Other Diagnoses Are Less Likely

  • IBS-D or functional diarrhea - These conditions do not cause elevated fecal calprotectin or CRP 5, 4. Fecal calprotectin has 100% sensitivity and 88% specificity for excluding IBD in primary care settings 5.

  • Microscopic colitis - While possible, the extremely elevated fecal calprotectin (>3000 μg/g) is more consistent with IBD than microscopic colitis, which typically shows more modest elevations.

  • Small intestinal bacterial overgrowth - Would not explain the markedly elevated inflammatory markers 4.

The constellation of chronic symptoms, dramatically elevated fecal calprotectin, elevated CRP, and negative celiac serology in a 16-year-old makes IBD (most likely Crohn's disease) the diagnosis until proven otherwise by endoscopy 1, 2.

References

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