SeHCAT (23-seleno-25-homotaurocholic acid, selenium homocholic acid taurine, or tauroselcholic acid) is a drug used in a clinical test to diagnose bile acid malabsorption.

It is a taurine-conjugated bile acid analog synthesized for use as a radiopharmaceutical to investigate in vivo the enterohepatic circulation of bile salts.

The gamma-emitter 75Se is incorporated into the SeHCAT molecule.

The  retention in the body or the loss of this compound into the feces could be studied using a standard gamma camera.

SeHCAT is absorbed from the gut and excreted into the bile at the same rate as cholic acid, one of the major natural bile acids.

SeHCAT undergoes secretion into the biliary tree, gallbladder and intestine in response to food, and is reabsorbed efficiently in the ileum.

SeHCAT kinetics are similar to natural bile acids.

This test has replaced 14C-labeled glycocholic acid breath tests and fecal bile acid measurements, in the routine clinical investigation of malabsorption.

A capsule containing radiolabelled 75SeHCAT is taken orally with water, to ensure passage of the capsule into the gastrointestinal tract. 

The physical half life of 75Se is approximately 118 days.

The effective dose of radiation for an adult given 370 kBq of SeHCAT is 0.26 mSv.

The radiation exposure from an abdominal CT scan is quoted at 5.3 mSv.

The patient is scanned supine or prone with anterior and posterior acquisition from head to thigh 1 to 3 hours after taking the capsule. 

Scanning is repeated after 7 days. 

The percent retention of SeHCAT at 7 days is calculated. 

A 7-day SeHCAT retention value greater than 15% is considered to be normal, with values less than 15% signifying excessive bile acid loss, as found in bile acid malabsorption.

A half-life of greater than 2.8 days is normal.

The SeHCAT test is used to investigate patients with suspected bile acid malabsorption:  experiencing chronic diarrhea, often passing watery feces 5 to 10 times each day. 

When the ileum has been removed following surgery, or is inflamed by Crohn’s disease, the 7-day SeHCAT retention usually is abnormal, and most of these patients will benefit from treatment with bile acid sequestrants.

The enterohepatic circulation of bile acids is reduced inpatients with ileal abnormalities.

The  normal bile acid retention exceeds 95%, only a small degree of change is needed to cause bile diarrhea.

Bile acid malabsorption can also be secondary to  other factors including: cholecystectomy, vagotomy and other disorders affecting intestinal motility or digestion such as radiation enteritis, celiac disease, and small intestinal bacterial overgrowth.

Chronic diarrhea, an abnormal SeHCAT retention and a response to bile acid sequestrants, in the absence of other disorders of the intestine, is characteristic of idiopathic bile acid malabsorption.

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