What diagnostic tests are needed for a patient with abdominal pain and diarrhea?

Diagnostic Testing for Abdominal Pain and Diarrhea

The diagnostic approach depends critically on whether the diarrhea is acute (< 4 weeks) or chronic (≥ 4 weeks), with acute cases requiring minimal testing unless severe features are present, while chronic cases warrant systematic laboratory and endoscopic evaluation.

Acute Diarrhea with Abdominal Pain (< 4 weeks duration)

When Testing is NOT Needed

• Most cases of uncomplicated acute diarrhea do not require diagnostic testing, as they are typically self-limited viral or bacterial infections 1
• Testing is unnecessary in mild cases without fever, bloody stools, severe dehydration, or immunocompromise 1

When Testing IS Required

Stool testing should be performed when any of the following are present: 1

• Bloody or mucoid stools (suggests Shiga toxin-producing E. coli, Shigella, Salmonella, Campylobacter, or Entamoeba histolytica) 1
• Fever with severe illness (though STEC patients are typically afebrile at presentation) 1
• Severe abdominal pain (consider STEC, Salmonella, Shigella, Campylobacter, Yersinia, or C. difficile) 1
• Immunocompromised state (requires broader testing including viral studies, parasites, Cryptosporidium, Cyclospora, Cystoisospora, microsporidia, and in AIDS patients, MAC and CMV) 1
• Recent antibiotic use within 8-12 weeks (test for C. difficile) 1
• Possible outbreak setting (multiple people affected, shared meal exposure) 1
• Persistent symptoms ≥ 14 days in travelers (evaluate for intestinal parasites) 1

Imaging for Acute Abdominal Pain

• Plain abdominal radiography during acute episodes to exclude bowel obstruction and other pathology 1
• Computed tomography (CT) is recommended for evaluating right or left lower quadrant pain when serious pathology is suspected 1
• Ultrasonography is the initial test for right upper quadrant pain 1

Chronic Diarrhea with Abdominal Pain (≥ 4 weeks duration)

Initial Laboratory Testing (Primary Care Level)

The following tests should be performed initially in ALL patients with chronic diarrhea: 1, 2

• Complete blood count (to detect anemia suggesting organic disease) 1
• Erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP) (elevated in inflammatory bowel disease, though limited utility) 1
• Serum electrolytes, liver function tests 1
• Iron studies, vitamin B12, folate 1
• Thyroid-stimulating hormone (TSH) (to exclude hyperthyroidism) 1, 2
• Celiac disease serology: IgA tissue transglutaminase antibody with total IgA level (most important screening test for the most common small bowel enteropathy) 1, 2

Stool Testing for Chronic Diarrhea

• Fecal calprotectin to distinguish inflammatory from non-inflammatory causes (though role debated without alarm features) 2, 3
• Stool for ova and parasites only if travel to or immigration from endemic regions 1, 3
• Stool for Giardia (common parasitic cause of chronic diarrhea) 3
• Fecal elastase if pancreatic insufficiency suspected (preferred over 3-day fecal fat, which is unreliable) 1

Endoscopic Evaluation

Age-stratified colonoscopy approach: 1, 2

• Age ≥ 50 years: Full colonoscopy with biopsies (due to colorectal cancer risk) 1
• Age 45-50 years: Consider colonoscopy based on family history and risk factors 1
• Age < 45 years: Flexible sigmoidoscopy with biopsies (diagnostic yield not substantially different from colonoscopy in this age group without alarm features) 1

Colonic biopsies should be obtained even if mucosa appears normal to exclude microscopic colitis, which affects 10% of patients with chronic watery diarrhea 2

Upper endoscopy with distal duodenal biopsies is indicated if: 1

• Celiac serology is positive (required for definitive diagnosis before treatment) 4
• Suspected small bowel malabsorption despite negative celiac serology 1
• Clinical features suggest small bowel disease 1

When Imaging is Indicated

• CT abdomen/pelvis should be performed if alarm features develop (weight loss, severe pain, signs of obstruction), elevated inflammatory markers suggest IBD, or concern for complications 1, 2, 4
• CT angiography without delay if acute mesenteric ischemia suspected (sudden severe pain, especially with atrial fibrillation, recent MI, or low-flow states) 1

Critical Pitfalls to Avoid

• Do not perform multiple-pathogen nucleic acid amplification tests (gastrointestinal panels) without clinical context, as these detect DNA, not necessarily viable organisms or clinically significant infections 1
• Do not test for celiac disease after starting a gluten-free diet, as this causes false-negative results 4
• Do not rely on fecal fat testing, as it is unreliable in clinical practice and does not discriminate between small bowel and pancreatic malabsorption 1
• Do not delay CT angiography in suspected mesenteric ischemia despite acute kidney injury, as delayed diagnosis doubles mortality every 6 hours 1
• Do not attribute symptoms to IBS without excluding celiac disease first, as celiac disease is a common mimic 2, 3
• Do not order hydrogen breath tests for small intestinal bacterial overgrowth in typical IBS presentations, as sensitivity is only ~60% and specificity ~75% 1, 3

Special Considerations

For diarrhea-predominant symptoms specifically: 1

• Consider lactose/dextrose H2 breath test if regular dairy consumption, especially in high-risk ethnic groups 1, 3
• Consider bile acid malabsorption testing (SeHCAT scan or serum 7α-hydroxy-4-cholesten-3-one) if symptoms persist despite negative workup 1, 3
• Small bowel biopsies for giardia or microscopic colitis may be indicated 1

For pain-predominant symptoms: 1

• Plain radiography during acute episodes to exclude obstruction 1
• Consider empirical antispasmodic trial if imaging negative 1