Se HCAT - SeHCAT Test Procedure

SeHCAT Test Procedure

A capsule containing radiolabelled 75SeHCAT (with 370kBq of Selenium-75 and less than 0.1 mg SeHCAT) is taken orally. The physical half life of 75Se is approximately 118 days; activity is adjusted to a standard reference date. Patients are often fasted before taking the capsule and swallow water to ensure passage of the capsule into the gastrointestinal tract. The effective dose of radiation for an adult given 370kBq of SeHCAT is 0.26mSv. (For comparison, the radiation exposure from an abdominal CT scan is quoted at 5.3mSv and annual background exposure in the UK 1-3mSv.) Measurements were originally performed with a whole body counter but are usually performed now with an uncollimated gamma camera. 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. Background values are subtracted and care must be taken to avoid external sources of radiation in a nuclear medicine department.

From these measurements, 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% signifing excessive bile acid loss, as found in bile acid malabsorption.

With more frequent measurements, it is possible to calculate SeHCAT retention whole body half-life; this is not routinely measured in a clinical setting. A half-life of greater than 2.8 days has been quoted as normal.