What's the next step for a patient with 1 month of diarrhea (loose stool) and new abdominal pain, with negative stool cultures and normal bloodwork, considering imaging studies?

Colonoscopy with Biopsies is the Next Step for Chronic Diarrhea with New Abdominal Pain

For a patient with one month of loose stool (diarrhea) and new abdominal pain with negative stool cultures and normal bloodwork, colonoscopy with biopsies is the recommended next imaging study.

Diagnostic Approach Rationale

Chronic diarrhea (defined as lasting more than 4 weeks) with new abdominal pain requires thorough evaluation, even when initial laboratory tests are normal. The presentation suggests possible inflammatory bowel disease, microscopic colitis, or other structural abnormalities that require direct visualization and tissue sampling.

Why Colonoscopy First:

1. Direct visualization and sampling capability:

   • Colonoscopy allows direct visualization of the colonic mucosa and terminal ileum
   • Enables targeted biopsies of both normal and abnormal-appearing mucosa 1
   • Can identify microscopic colitis which presents with normal-appearing mucosa but abnormal histology 1

2. Diagnostic yield:

   • Colonoscopy with biopsies is strongly recommended for all patients with chronic diarrhea lasting more than one month 1
   • Biopsies should be taken from both right and left colon, even if mucosa appears normal 1
   • Terminal ileal intubation and biopsy should be performed when Crohn's disease is suspected 1

3. Guidelines support:

   • The American Gastroenterological Association recommends colonoscopy with biopsies for chronic diarrhea evaluation 2
   • European Society of Gastrointestinal Endoscopy and American College of Gastroenterology also recommend this approach 1

Biopsy Recommendations

When performing colonoscopy:

• Take biopsies from both right and left colon, even if mucosa appears normal (to detect microscopic colitis)
• Perform terminal ileal intubation and biopsy when possible
• Obtain at least 2 biopsies from each segment of the colon

Cross-Sectional Imaging Considerations

If colonoscopy is negative or contraindicated, or if small bowel disease is suspected:

1. CT Enterography:

   • Preferred for evaluating small bowel disease 2
   • Excellent for detecting inflammation, strictures, fistulae, and abscesses 2
   • The WSES-AAST guidelines recommend IV contrast-enhanced CT scan to exclude intestinal perforation, stenosis, bleeding, and abscesses 2

2. MR Enterography:

   • Alternative to CT with no radiation exposure 1
   • Particularly useful for young patients who may need repeated imaging 3
   • Better at differentiating fibrotic from inflammatory strictures 2

3. Ultrasound:

   • Can be used as point-of-care assessment when CT is not available 2
   • Useful for detecting free fluid, abscesses, or intestinal distention 2
   • Less sensitive than CT or MRI for comprehensive evaluation 2

Additional Diagnostic Considerations

If initial colonoscopy is negative:

1. Upper endoscopy with duodenal biopsies:

   • To evaluate for celiac disease if serology was not performed 2
   • Can identify other causes of malabsorption

2. Video capsule endoscopy:

   • Consider if small bowel disease is suspected after negative colonoscopy 2
   • Not recommended as first-line investigation 2
   • Contraindicated if strictures are suspected 2

Pitfalls to Avoid

1. Premature diagnosis of IBS:

   • Do not diagnose IBS without excluding inflammatory and neoplastic conditions 1
   • Rome II criteria for IBS should only be applied after organic causes are ruled out 2

2. Inadequate biopsies:

   • Failing to take biopsies from normal-appearing mucosa can miss microscopic colitis 1
   • Both right and left colon biopsies are essential 1

3. Relying solely on imaging without histology:

   • Cross-sectional imaging alone cannot detect microscopic inflammation 2
   • Tissue diagnosis is often necessary for definitive diagnosis and treatment planning 1

4. Overlooking medication causes:

   • Many medications can cause chronic diarrhea and should be reviewed 1

Conclusion

For a patient with one month of diarrhea and new abdominal pain with negative initial workup, colonoscopy with biopsies is the most appropriate next step. This approach provides the highest diagnostic yield and allows for direct visualization and tissue sampling to guide appropriate treatment.