Diagnosing the true cause of someone's "gluten allergy" would allow them to start eating gluten again. Since gluten is pretty much everywhere, I'd say that'd be a nice quality of life improvement.
Low-cost screenings, such as 23&Me mail-in swab tests, elimination diets, or common blood and urine tests are a great way to start building up that objective evidence without first doing harm to your patient's wallet.