What is the most likely diagnosis and recommended next diagnostic and therapeutic steps for a patient with markedly elevated fecal calprotectin (~1589 µg/g), presence of fecal leukocytes, and a negative infectious stool workup (negative Clostridioides difficile PCR, negative bacterial culture, and no ova/parasites)?

Inflammatory Bowel Disease (Crohn's Disease or Ulcerative Colitis)

This patient most likely has inflammatory bowel disease (IBD), and you should proceed directly to ileocolonoscopy with biopsies to establish the diagnosis and guide treatment. The markedly elevated fecal calprotectin of 1589 µg/g with fecal leukocytes and negative infectious workup strongly indicates active intestinal inflammation requiring endoscopic evaluation 1, 2.

Diagnostic Reasoning

The fecal calprotectin level of 1589 µg/g is dramatically elevated (>10-fold above the 150 µg/g threshold) and essentially rules in significant mucosal inflammation. When combined with:

• Presence of fecal neutrophils (indicating active inflammation)
• Negative C. difficile PCR
• Negative bacterial pathogens (Salmonella, Shigella, Campylobacter, E. coli O157:H7)
• Negative ova and parasites
• Negative H. pylori antigen

This constellation makes IBD the overwhelming diagnostic probability 2.

Why Endoscopy is Essential Now

Do not attempt empiric treatment without endoscopic confirmation. The AGA guidelines specifically state that in patients with elevated biomarkers >150 µg/g, endoscopic assessment should be performed rather than empiric treatment adjustment 2. This is even more critical in a new diagnosis scenario where:

• You need tissue diagnosis to differentiate Crohn's disease from ulcerative colitis
• Treatment strategies differ significantly between these conditions
• Histopathology is required to exclude mimics (drug-induced colitis, ischemic colitis, etc.)
• Disease extent and severity guide therapeutic decisions

Specific Endoscopic Protocol

Perform ileocolonoscopy with the following biopsy protocol 1:

• At least 2 biopsies from terminal ileum
• At least 2 biopsies from each of 4-5 different colonic segments (cecum, ascending, transverse, descending, sigmoid)
• At least 2 biopsies from rectum
• Clearly label each biopsy site

If the patient has upper GI symptoms (dysphagia, epigastric pain, nausea/vomiting), also perform esophagogastroduodenoscopy with biopsies 1. However, routine upper endoscopy is not indicated in adults without upper GI symptoms.

Fecal Calprotectin Interpretation Context

Your patient's calprotectin level of 1589 µg/g far exceeds diagnostic thresholds:

• >150 µg/g reliably indicates moderate-to-severe endoscopic inflammation in symptomatic patients 2
• Levels >250 µg/g have even higher specificity for active disease 2
• At 1589 µg/g, the positive predictive value for significant mucosal inflammation approaches 95%

Research shows fecal calprotectin has excellent diagnostic accuracy for IBD, with sensitivity of 96.6% and specificity improving with higher cutoffs 3, 4. The negative predictive value is particularly strong—meaning normal calprotectin effectively rules out IBD—but your patient has the opposite scenario with extremely elevated levels.

Critical Pitfall to Avoid

Do not delay endoscopy to repeat biomarkers or wait for symptom evolution. While the AGA suggests that repeat biomarker testing in 3-6 months may be reasonable in asymptomatic patients with borderline elevations 2, this does NOT apply to your patient with:

• Calprotectin >10-fold elevated
• Fecal leukocytes present
• Presumably symptomatic (given the extensive workup performed)

The British Society of Gastroenterology guidelines emphasize that fecal calprotectin >250 µg/g warrants endoscopic/radiological evaluation and discussion about therapy optimization 1.

Additional Considerations

While awaiting endoscopy, check:

• Complete blood count (anemia, thrombocytosis suggest IBD) 3
• Albumin (hypoalbuminemia increases IBD likelihood) 3
• CRP (though less sensitive than calprotectin for intestinal inflammation)
• Iron studies if anemia present

These help risk-stratify but should not delay endoscopy.

Small Bowel Evaluation

If ileocolonoscopy shows isolated terminal ileal disease or is entirely normal despite the markedly elevated calprotectin, consider small bowel imaging (MR enterography or CT enterography) or video capsule endoscopy to evaluate for proximal small bowel Crohn's disease 1, 5. Recent data shows fecal calprotectin sensitivity increases from proximal to distal small bowel, so very high levels suggest distal small bowel or colonic involvement 5.

The bottom line: Schedule ileocolonoscopy immediately—this patient needs tissue diagnosis before initiating immunosuppressive therapy for presumed IBD.