What is the recommended test for diagnosing bile acid malabsorption in patients with chronic diarrhea?

Diagnostic Testing for Bile Acid Malabsorption in Chronic Diarrhea

For patients with chronic diarrhea, the 75-selenium homocholic acid taurine (SeHCAT) scan is the recommended gold standard test for diagnosing bile acid malabsorption, though in the United States where SeHCAT is unavailable, measurement of total bile acids in a 48-hour stool collection or serum fibroblast growth factor 19 (FGF19) are the preferred diagnostic options. 1, 2

First-Line Diagnostic Tests

In Europe and where available:

• SeHCAT scan: Gold standard with highest diagnostic yield
  • Interpretation of retention values:
    • 10-15% = mild bile acid loss
    • 5-10% = moderate bile acid loss
    • 0-5% = severe bile acid loss 2
  • Has demonstrated excellent predictive value for response to bile acid sequestrants 3

In North America (where SeHCAT is unavailable):

1. 48-hour stool collection for total bile acids

   • Threshold: >2300 μmol/48 hours indicates bile acid malabsorption
   • Documents increased fecal bile acid excretion 1

2. Serum fibroblast growth factor 19 (FGF19)

   • Measures defective feedback of bile acid synthesis
   • Low levels suggest bile acid malabsorption 1, 4

3. Serum 7α-hydroxy-4-cholesten-3-one (C4)

   • Requires fasting sample
   • Diagnostic threshold: >47.1 ng/mL
   • 95% negative predictive value compared to SeHCAT 2
   • Measures bile acid synthesis 4

When Testing is Not Available

The American Gastroenterological Association acknowledges that when specific bile acid malabsorption tests are not available, an empiric trial of bile acid binders may be considered, with clinical response suggesting excess bile acids as the cause for diarrhea 1. However, the British Society of Gastroenterology strongly recommends against empiric trials without diagnostic testing 2.

Clinical Considerations

• Bile acid malabsorption is frequently underdiagnosed, particularly in patients labeled as having IBS-D, where up to 30% may actually have bile acid malabsorption 2, 5
• Highest prevalence of bile acid malabsorption is seen in patients with:
  • Terminal ileal resection or right hemicolectomy (nearly 100% positive) 6, 5
  • Previous cholecystectomy (increased risk by 2.5 times) 5
  • Post-infectious diarrhea 1

Testing Algorithm

1. Rule out other causes of chronic diarrhea first:

   • Celiac disease (IgA-tTG)
   • Inflammatory markers (fecal calprotectin)
   • Giardia testing 1, 2

2. Select appropriate bile acid malabsorption test based on availability:

   • If SeHCAT available: Perform SeHCAT scan
   • If SeHCAT unavailable: Order 48-hour stool collection for bile acids or serum FGF19

3. Interpret results:

   • Positive test: Initiate bile acid sequestrant therapy
   • Negative test: Investigate other causes of diarrhea

Important Caveats

• Even with negative testing, some patients with chronic diarrhea may still respond to bile acid sequestrants 7
• Lack of response to cholestyramine does not exclude bile acid malabsorption; alternative sequestrants like colesevelam should be considered 2
• The diagnostic yield of testing is particularly high in patients with risk factors (ileal resection, cholecystectomy), but testing should still be considered in patients with unexplained chronic diarrhea without risk factors, as 37.5% may test positive 5

The American Gastroenterological Association conditionally recommends testing for bile acid diarrhea in patients with chronic diarrhea, reflecting the low quality of evidence but recognizing the importance of identifying this treatable condition 1.