Which gastrointestinal conditions cause chronic, persistent fatigue? can tests assist a diagnosis and, if positive, what treatments are effective?

Lots...: Here's just a few: GI bleeding conditions, inflammatory bowel disease, infection (e.g. chronic hepatitis C, invasive enteritis), parasite infestation, chronic liver disease, cancers (solid tumors, gut lymphoma, etc.), GI-related endocrinopathies, many more. Yes, you likely will benefit from testing, and then from result-directed treatment.

Many causes to Fatig: Fatigue can be caused by many possible reasons. We need a detailed history from you including: how long have you had this problem? What kind of job/work do you do? Have you lost any blood recently or have any other reason to be anemic.You should get your blood tested to find out your hemoglobin level. if it is low(Anemia) you will need to undergo further tests. Are you able to sleep well? ? Hours?