What is the next best step to investigate the cause of floating stool in a patient with normal lab results and concern for pancreatic cancer?

Reassurance and Initial Testing for Floating Stool

In this 41-year-old woman with isolated floating stool, normal labs (including normal amylase and lipase), and no alarm features, the next best step is to order a fecal elastase-1 test to evaluate for pancreatic exocrine insufficiency, while providing reassurance that pancreatic cancer is highly unlikely given her clinical presentation.

Why Pancreatic Cancer is Unlikely

This patient's presentation does not fit the typical profile of pancreatic cancer:

• Pancreatic cancer typically presents with pain, weight loss, and jaundice 1, 2. This patient has none of these cardinal features.
• Normal amylase and lipase effectively rule out acute pancreatic pathology 3. While these enzymes are not sensitive for chronic pancreatic insufficiency, their normal values in the context of no other symptoms make active pancreatic disease unlikely 4.
• Floating stool alone, when well-formed and normal color, is a non-specific finding that can occur with increased gas content or dietary factors, not necessarily fat malabsorption 4.
• The absence of weight loss is particularly reassuring, as pancreatic cancer patients typically present with significant weight loss and anorexia 1, 2.

Appropriate Diagnostic Approach

First-Line Testing: Fecal Elastase-1

Order a fecal elastase-1 test as the most appropriate initial diagnostic test 4, 5:

• Must be performed on semi-solid or solid stool (not liquid/watery stool, which gives false positives) 4, 5
• Interpretation 4, 5:
  • <100 μg/g = good evidence of pancreatic exocrine insufficiency (EPI)
  • 100-200 μg/g = indeterminate

  • 200 μg/g = normal

• Can be performed while on pancreatic enzymes if already started 4
• This test is preferred over therapeutic trials of pancreatic enzymes, which are unreliable for diagnosis 4

Why Not Other Tests?

• Serum enzymes (lipase, amylase, trypsin) are not useful for diagnosing chronic pancreatic insufficiency, as they remain normal until disease is far advanced (>90% pancreatic destruction) 4, 3
• Fecal fat testing is poorly reproducible, unpleasant, and generally not practical for routine use 4, 5
• Cross-sectional imaging (CT, MRI, endoscopic ultrasound) cannot diagnose EPI, though they can identify structural pancreatic disease 4, 5

Additional Considerations

Evaluate for Alternative Causes

While awaiting fecal elastase results, consider:

• Small intestinal bacterial overgrowth (SIBO), which can cause floating stool and gas-related symptoms 4. The British Society of Gastroenterology recommends hydrogen-methane breath testing if available, or empiric treatment with rifaximin 550 mg twice daily for 1-2 weeks 4
• Dietary factors: Assess fiber intake (too much or too little can cause symptoms) 4
• Celiac disease: Though less likely with normal labs, fecal elastase <500 μg/g can occur with untreated celiac disease 4

If Fecal Elastase is Abnormal (<100 μg/g)

Then consider cross-sectional imaging to evaluate for underlying pancreatic pathology (chronic pancreatitis, structural abnormalities, or malignancy) 4, 5. However, given this patient's benign presentation, imaging would primarily be for completeness rather than cancer concern.

Common Pitfalls to Avoid

• Do not order imaging first without functional testing, as imaging cannot diagnose EPI 4, 5
• Do not start empiric pancreatic enzyme therapy as a diagnostic test, as response is unreliable and the approach is not recommended 4
• Do not perform fecal elastase on liquid/watery stool, as this causes false positive results 4, 5
• Do not over-investigate based on patient anxiety alone when clinical features are reassuring 1, 2