What tests should be ordered for a patient presenting with malabsorption?

Diagnostic Tests for Malabsorption Syndrome

Patients with suspected malabsorption should undergo a stepwise diagnostic approach starting with blood tests, stool tests, and serological testing for celiac disease, followed by more specific tests based on clinical suspicion of the underlying cause. 1, 2

Initial Laboratory Screening

Blood Tests

• Complete blood count (CBC) to assess for anemia
• Comprehensive metabolic panel including:
  • Electrolytes
  • Liver function tests
  • Albumin (low levels suggest protein malabsorption)
• Iron studies including ferritin
• Vitamin B12 and folate levels
• Thyroid function tests
• Calcium, magnesium, and phosphorus levels
• Fat-soluble vitamin levels (A, D, E, K) 2

Stool Tests

• Fecal elastase-1: First-line test for pancreatic exocrine insufficiency
  • Values <200 μg/g suggest pancreatic insufficiency
  • Values <100 μg/g indicate severe pancreatic insufficiency 1, 2
• Fecal calprotectin: To exclude colonic inflammation in patients under 40 years of age 1
• Fecal immunochemical test (FIT): To exclude occult gastrointestinal bleeding 1

Serological Tests

• Celiac disease screening:
  • Anti-tissue transglutaminase (tTG) antibodies (IgA)
  • Anti-endomysial antibodies (EMA)
  • Total IgA levels (to rule out IgA deficiency)
  • Consider IgG-based testing if IgA deficiency is present 1

Second-Line Tests Based on Clinical Suspicion

For Suspected Small Bowel Disease

• Upper endoscopy with duodenal biopsies: Gold standard for diagnosing celiac disease and other enteropathies 1, 2
• MR enterography: Recommended for evaluation of small bowel abnormalities 1
• Video capsule endoscopy (VCE): For assessing small bowel abnormalities when MR enterography is inconclusive 1

For Suspected Pancreatic Disease

• MRI of the pancreas: Preferred over CT for assessing structural abnormalities in suspected chronic pancreatitis 1
• Endoscopic ultrasound (EUS): For detailed evaluation of pancreatic structure 2

For Suspected Bile Acid Malabsorption

• 75SeHCAT scan: Gold standard for diagnosing bile acid malabsorption
  • Values <15% at 7 days suggest bile acid malabsorption 1
• Serum 7α-hydroxy-4-cholesten-3-one: Alternative to SeHCAT where available 1

For Suspected Small Bowel Bacterial Overgrowth

• Empirical trial of antibiotics: Recommended over breath testing due to limited sensitivity and specificity of breath tests 1
• Jejunal aspirate culture: Gold standard but invasive and not routinely performed 1, 2

For Suspected Carbohydrate Malabsorption

• Hydrogen breath testing: For suspected lactose maldigestion 1
• Trial of dietary elimination: Alternative to breath testing 1

Tests No Longer Recommended

• Quantitative 72-hour fecal fat collection: Poor reproducibility, unpleasant, and non-diagnostic 1
• D-xylose testing: Poor performance in routine practice 1
• Small bowel barium studies: Poor sensitivity and specificity 1
• PABA testing: Not recommended for pancreatic function assessment 1

Common Pitfalls to Avoid

1. Relying solely on clinical symptoms: Steatorrhea may be absent even in significant malabsorption 1

2. Overlooking IgA deficiency when testing for celiac disease: Consider total IgA levels or IgG-based testing 1

3. Misinterpreting fecal elastase results: False positives can occur in non-pancreatic causes of malabsorption; values may be normal in mild pancreatic insufficiency 3

4. Single sample testing for fecal elastase: Significant intra-patient variability exists, especially in borderline cases 4

5. Premature empirical treatment: Making a definitive diagnosis before initiating treatment is important for proper management 1

6. Failure to consider microscopic colitis: Requires colonoscopy with biopsies of right and left colon (not just rectum) 1

By following this systematic approach to testing, clinicians can efficiently diagnose the underlying cause of malabsorption and initiate appropriate treatment to improve patient outcomes.