IBS and IBD

SeHCAT scan

Also called: bile acid retention test

SeHCAT (selenium-75 homocholic acid taurine) is a nuclear medicine scan that measures how much bile acid your body retains over a week. It diagnoses bile acid diarrhoea, which mimics IBS-D and affects up to a third of people diagnosed with diarrhoea-predominant IBS. The result determines whether bile acid binders (colestyramine, colesevelam) will work, and they often work dramatically.

Why this matters

Around 25-30% of people diagnosed with IBS-D actually have bile acid diarrhoea (Wedlake 2009 meta-analysis, Aliment Pharmacol Ther). The condition is treatable. Bile acid binders (colestyramine, colesevelam) work in roughly two thirds of confirmed cases, with response rates rising as SeHCAT retention falls. But the diagnosis is missed for years because most people are told they have IBS without ever being tested. SeHCAT is the gold-standard test.

How the scan works

  1. You swallow a capsule of SeHCAT (a synthetic bile acid labelled with a tiny dose of radioactive selenium-75).
  2. On day 1, a gamma camera measures how much SeHCAT is in your body (baseline).
  3. On day 7, the same scan measures how much remains.
  4. The 7-day retention figure tells you how well your gut is reabsorbing bile acids.
  5. Total radiation dose is small (around 0.26 mSv, similar to a few weeks of natural background radiation) and considered safe in adults.

Reading the result

  • Over 15 percent retention: normal. Bile acid diarrhoea unlikely.
  • 10 to 15 percent: mild bile acid diarrhoea. Treatment can still help.
  • 5 to 10 percent: moderate. Treatment usually works well.
  • Under 5 percent: severe. Treatment almost always works.
  • Lower retention = more bile acid escaping into the colon = more diarrhoea.

What to do with a positive result

  1. Colestyramine (Questran): 4 g once or twice a day, mixed in water. Cheap, effective, but tastes unpleasant.
  2. Colesevelam (Cholestagel): tablet form, fewer side effects than colestyramine, more expensive.
  3. Take other medications at least 1 hour before, or 4-6 hours after, colestyramine (it binds them too).
  4. Reduce fatty meals during flares.
  5. Investigate underlying cause: ileal disease (Crohn's, surgery), ileal resection, post-cholecystectomy.

Common questions

Is SeHCAT available on the NHS?
Yes, but access varies. Some Trusts have it readily; others require gastroenterology referral. Worth asking your GP if you have IBS-D-like symptoms, especially with watery, urgent or post-meal diarrhoea.
Are there alternatives to SeHCAT?
Less specific. Some clinicians do a therapeutic trial of colestyramine, if it works, you have bile acid diarrhoea. Faster but less rigorous. C4 blood test (a bile acid synthesis marker) is sometimes used but not as accurate.
Is the radiation dose safe?
Yes. SeHCAT delivers around 0.26 mSv over the test, similar to several months of natural UK background radiation (~2 mSv/year). It's higher than a single chest X-ray (~0.1 mSv) but well below a CT abdomen (~7.7 mSv). Safe in adults; usually avoided in pregnancy.
How quickly will treatment work?
Often quickly. Colestyramine starts binding bile acids straight away, and most patients with severe bile acid diarrhoea respond to treatment, though degree and speed of improvement vary.

Sources

The Book of Suna is general information, not medical advice. It is not a substitute for talking to a qualified healthcare professional about your own situation.