Blood test has poor reliability, and you have to eat gluten to get it which makes many people too sick to even get tested

The gold standard is a biopsy of the duodenum but again you need to be eating like 20g of gluten a day for a few weeks and many people get too sick after even a few milligrams

Neither diagnoses NCGS, non Celiac gluten sensitivity. The only test is to stop eating gluten.