What labs should be considered for a patient with ongoing abdominal pain?

Laboratory Tests for Ongoing Abdominal Pain

For patients with ongoing abdominal pain, initial laboratory evaluation should include a complete blood count, C-reactive protein, serum chemistries, and stool testing for occult blood, with additional tests guided by clinical presentation and suspected etiology. 1

Initial Laboratory Evaluation

• Complete blood count (CBC) to assess for leukocytosis (infection/inflammation) or anemia (potential gastrointestinal bleeding) 1
• C-reactive protein (CRP) as a marker of inflammation, particularly useful in younger patients 1
• Serum chemistries and albumin to evaluate for metabolic abnormalities and nutritional status 1
• Stool Hemoccult testing to screen for occult gastrointestinal bleeding 1
• Erythrocyte sedimentation rate (ESR), especially in younger patients with suspected inflammatory conditions 1

Additional Tests Based on Clinical Presentation

For Diarrhea-Predominant Symptoms:

• Stool for ova and parasites, particularly in endemic areas or with relevant travel history 1
• Stool testing for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and STEC in patients with fever, bloody or mucoid stools 1
• Lactose/dextrose H2 breath test for suspected carbohydrate malabsorption 1
• Serologies for celiac disease 1

For Right Upper Quadrant Pain:

• Liver function tests including ALT, AST, alkaline phosphatase, and bilirubin 2, 3
• Hepatobiliary markers for suspected gallbladder disease 3
• Lipase and amylase for suspected pancreatic disease 3

For Lower Quadrant or Pelvic Pain:

• Pregnancy test for all women of reproductive age (mandatory before imaging) 2, 3
• Urinalysis to evaluate for urinary tract infection or nephrolithiasis 3

For Suspected Inflammatory Bowel Disease:

• Fecal calprotectin as a marker of intestinal inflammation 1
• CRP and ESR to assess inflammatory activity 1
• Complete blood count with differential to evaluate for anemia and leukocytosis 1

For Suspected Mesenteric Ischemia:

• Lactate levels (elevated in bowel ischemia) 2, 3
• D-dimer (may be elevated, though not definitive) 2

Special Considerations

• Blood cultures should be obtained in patients with fever, signs of septicemia, or immunocompromised status 1
• In immunocompromised patients, especially those with AIDS and persistent diarrhea, additional testing for Cryptosporidium, Cyclospora, Cystoisospora, microsporidia, Mycobacterium avium complex, and cytomegalovirus should be considered 1
• For patients with persistent symptoms (>14 days), particularly travelers, testing for intestinal parasitic infections is recommended 1
• For patients with recent antibiotic use (within 8-12 weeks), testing for C. difficile should be performed 1

Laboratory Testing Algorithm

1. Initial evaluation: CBC, CRP, serum chemistries, stool Hemoccult 1
2. If fever present: Add blood cultures and consider chest radiograph 3
3. Based on pain location:
   • Right upper quadrant: Add liver function tests and hepatobiliary markers 2, 3
   • Lower quadrants: Consider stool studies for infectious causes 1
   • Diffuse pain: Consider broader testing including lactate for suspected ischemia 2
4. Based on predominant symptoms:
   • Diarrhea: Add stool studies for infectious causes 1
   • Constipation: Limited additional testing needed initially 1

Common Pitfalls

• Over-reliance on a single laboratory parameter; no single test is sufficiently accurate to identify all causes of abdominal pain 2
• Failure to consider age-specific concerns (elderly patients may have normal laboratory tests despite serious pathology) 3
• Delaying appropriate imaging when indicated by laboratory abnormalities 3
• Repeating diagnostic tests in patients with established functional disorders without new concerning symptoms 4

Remember that laboratory tests should be interpreted in the context of the clinical presentation, and negative laboratory results do not exclude serious pathology, particularly in elderly patients or those with atypical presentations 3.