Principle of the SeHCAT Test
The SeHCAT test measures bile acid malabsorption by administering an oral dose of selenium-75-labeled synthetic bile acid (75Se-homotaurocholic acid) and quantifying whole-body retention at 7 days using a gamma camera, with retention <15% indicating abnormal bile acid loss. 1
Test Methodology
The patient ingests a capsule containing 75Se-homotaurocholic acid, a synthetic taurine-conjugated bile acid analog that resists bacterial degradation and passive absorption in the small intestine. 1
Gamma camera imaging is performed on day 1 (baseline) and day 7 to measure retained radioactivity over the whole body or abdomen. 1, 2
The 7-day retention percentage is calculated by comparing day 7 activity to day 1 activity, providing an indirect measure of bile acid absorption and fecal excretion. 1
Physiological Basis
Normally, >90-95% of bile acids are actively reabsorbed in the terminal ileum through the apical sodium-dependent bile salt transporter and recycled via the enterohepatic circulation. 1
The synthetic 75Se-homotaurocholic acid mimics natural conjugated bile acids but is resistant to bacterial deconjugation, ensuring that measured retention reflects true ileal absorption rather than colonic bacterial metabolism. 1
When ileal absorption is impaired, excess bile acids reach the colon where they stimulate fluid, mucus, and sodium secretion while increasing motility, causing secretory diarrhea. 1
Interpretation Thresholds
Normal retention: ≥15% at 7 days 1
Diagnostic Performance
The SeHCAT test demonstrates 89% sensitivity and 100% specificity for bile acid malabsorption, with a pooled positive likelihood ratio of 9.5. 1
SeHCAT retention <5% predicts nearly 100% response to cholestyramine therapy, while retention 5-10% predicts approximately 38% response, and retention 10-15% shows minimal therapeutic response. 1, 3, 2
In patients with terminal ileal resection, 100% demonstrate abnormal SeHCAT retention, with the majority showing severe malabsorption (<5%). 1, 2, 4
Clinical Context and Indications
SeHCAT testing should be ordered after excluding infection, inflammatory bowel disease, celiac disease, and medication-induced causes, particularly when risk factors for bile acid malabsorption are present (terminal ileal disease/resection, post-cholecystectomy, post-radiation therapy, idiopathic chronic diarrhea). 1, 3
Approximately 28-33% of patients with diarrhea-predominant IBS or idiopathic chronic diarrhea have bile acid malabsorption detected by SeHCAT. 1, 3
The test is positioned as a second-line investigation after basic screening (CBC, CRP, IgA-tTG, fecal calprotectin, Giardia testing). 3
Practical Limitations
SeHCAT requires administration of a radioactive isotope, dedicated gamma camera infrastructure, and two patient visits (days 1 and 7). 1
The test is not available in the United States and is only performed in specific countries (primarily Europe and Canada). 1
When SeHCAT is unavailable, serum C4 (7α-hydroxy-4-cholesten-3-one) can serve as an alternative, with levels >47.1 ng/mL indicating bile acid diarrhea, though C4 has lower sensitivity (40%) compared to SeHCAT. 1, 3
Common Pitfalls
SeHCAT measures only a single 7-day time point, unlike continuous fecal bile acid measurement, but this single measurement correlates well with clinical outcomes. 1
Patients with retention 10-15% often have functional IBS rather than true bile acid malabsorption and show poor response to bile acid sequestrants, so treatment should be reserved for retention <10%. 1, 3
In patients with extensive ileal resection (>100 cm), the primary problem shifts from bile acid diarrhea to fat malabsorption due to bile acid pool depletion, and bile acid sequestrants may worsen steatorrhea in this population. 3, 5