4.1 SeHCAT (GE Healthcare) is a radiopharmaceutical that is licensed for measuring bile acid pool loss and investigating bile acid malabsorption. It can also be used to assess ileal function, to investigate IBD and chronic diarrhoea, and to study the enterohepatic circulation.
4.2 SeHCAT consists ofa capsule containing a synthetic analogue of the natural conjugated bile acid tauroselcholic acid and 75Selenium (a gamma-emitter). The radionuclide tracer atom allows SeHCAT to be easily detected in a whole body scan using a standard gamma camera. The technology is used to test the function of the bowel by measuring how well the compound is retained or lost from the body into the faeces. SeHCAT is the only test currently used to diagnose bile acid malabsorption.
4.3 The SeHCAT test involves 2 scans, 1 week apart, carried out as outpatient appointments. During the first appointment, the SeHCAT capsule is administered orally and, once localised in the body (which takes 1–3 hours), the radionuclide tracer atom is detected in a whole-body baseline scan using a standard gamma camera. This gives an initial count that is used to provide a zero-time or 100% value. During the second appointment, the patient is scanned to produce a second count, and the retained activity is expressed as a percentage of the original value. In general practice, retention values of less than 15% have been considered abnormal and indicative of bile acid malabsorption. However, there is no definitive cut-off between normal and abnormal. In this assessment, 3 cut-off points were evaluated, 5%, 10% and 15%.
4.4 Although the models used in the assessment assumed no treatment differences based on the results of tests using SeHCAT, clinicians have used SeHCAT results to grade the severity of the bile acid malabsorption as follows:
retention values of 10–15% (mild bile acid malabsorption)
retention values of 5–10% (moderate bile acid malabsorption)
retention values of 0–5% (severe bile acid malabsorption).
4.5 There is no commonly used direct comparator for this diagnostic test. Current diagnostic options include analysis of a person's history of symptoms, investigations to exclude 'red flag' symptoms, and a variety of other diagnostic tests such as blood tests and lactose tolerance tests. The main comparator used in the assessment was the protocol of tests and clinical observations contained in the BSG guidelines for the investigation of chronic diarrhoea.