What investigations should be sent for a young patient presenting with diarrhea and constipation?

Diagnostic Investigations for Young Patients with Diarrhea and Constipation

For young patients presenting with both diarrhea and constipation, initial investigations should include screening blood tests, stool studies, and flexible sigmoidoscopy, followed by more targeted testing based on initial findings.

Initial Laboratory Investigations

• Complete blood count: To assess for anemia, infection, or inflammation 1, 2
• Inflammatory markers: ESR and C-reactive protein to detect inflammatory conditions 1, 2
• Basic metabolic panel: Electrolytes, urea, creatinine to assess hydration status 1
• Liver function tests: To rule out hepatobiliary causes 1, 2
• Thyroid function tests: Thyroid dysfunction can cause altered bowel habits 1, 2
• Iron studies, vitamin B12, folate: To assess for malabsorption 1, 2
• Calcium levels: Abnormalities can affect bowel motility 1
• Celiac disease serology: Anti-endomysium antibodies (EMA) or tissue transglutaminase (tTG) antibodies 1, 2

Stool Studies

• Stool cultures and microscopy: To rule out infectious causes 1, 2
• Stool for ova and parasites: Particularly important in endemic areas or with relevant travel history 1
• Fecal calprotectin: To detect inflammatory bowel disease 2
• Fecal immunochemical test: To screen for occult blood 2
• Clostridium difficile testing: Especially with history of antibiotic use 2
• Stool osmolality: If factitious diarrhea is suspected (value <290 mosmol/kg suggests dilution) 1
• Laxative screening: If factitious diarrhea is suspected, test for anthraquinones, bisacodyl, and phenolphthalein in urine, and magnesium and phosphate in stool 1

Endoscopic Evaluation

• Flexible sigmoidoscopy: First-line endoscopic investigation for patients under 45 years 1, 2

  • Should include biopsies of colonic mucosa even if normal appearing
  • Can detect microscopic colitis, Crohn's disease, ulcerative colitis, and melanosis coli
  • Studies show 99.7% of colonic pathology in young patients can be diagnosed with distal colon biopsies 1

• Colonoscopy: Indicated if:

  • Age >45 years
  • Alarm features present (weight loss, nocturnal symptoms, blood in stool)
  • Persistent symptoms despite normal initial investigations
  • Family history of colorectal cancer 1, 2
  • Terminal ileal intubation should be performed if Crohn's disease is suspected 2

• Upper endoscopy with duodenal biopsies: Consider if:

  • Celiac serology is positive or not performed
  • Other causes of malabsorption are suspected 2

Additional Investigations Based on Initial Findings

• Hydrogen breath tests: For suspected small bowel bacterial overgrowth (SBBO) or carbohydrate malabsorption 1

  • Glucose hydrogen breath test is recommended for SBBO
  • Lactose/dextrose H2 breath test for carbohydrate malabsorption

• 75Se-HCAT scan: To detect bile acid malabsorption if diarrhea predominates 1

• Imaging studies:

  • CT Enterography: For evaluating small bowel disease, detecting inflammation, strictures, fistulae 2
  • MR Enterography: Alternative to CT with no radiation exposure, better for young patients 2
  • Video capsule endoscopy: If small bowel disease is suspected after negative colonoscopy 2

Special Considerations

• Alternating diarrhea and constipation strongly suggests irritable bowel syndrome (IBS), especially with:

  • Abdominal pain relieved by defecation
  • Onset associated with change in stool frequency or form
  • Bloating or abdominal distention
  • Passage of mucus 1

• Factitious diarrhea should be considered in young patients with refractory symptoms, especially with:

  • Psychiatric history
  • Connection to health professions
  • Abnormal views on body shape 1

Common Pitfalls to Avoid

1. Premature diagnosis of IBS without excluding inflammatory and neoplastic conditions 2
2. Inadequate biopsies during endoscopy, which can miss microscopic colitis 2
3. Missing terminal ileal disease by not performing terminal ileal intubation when Crohn's disease is suspected 2
4. Overlooking medication causes of alternating diarrhea and constipation 2
5. Inadequate infection screening, especially in immunocompromised patients 2

Algorithm for Investigation

1. Start with screening blood tests and stool studies
2. If patient is <45 years without alarm features, proceed to flexible sigmoidoscopy with biopsies
3. If symptoms persist or alarm features present, proceed to colonoscopy with biopsies
4. Based on findings, consider specialized tests (breath tests, 75Se-HCAT scan, imaging studies)
5. If all tests negative but symptoms persist, consider functional bowel disorder and appropriate management

This systematic approach ensures thorough evaluation while minimizing unnecessary invasive procedures, focusing on the most likely diagnoses in young patients with alternating diarrhea and constipation.