The only way to confirm a celiac disease diagnosis is to have an intestinal biopsy.
A pathologist will assign a Modified Marsh Type to the biopsy findings.
A Type of 3 indicates symptomatic celiac disease. However, Types 1 and 2 may also indicate celiac disease.
Why an Intestinal Biopsy?
An intestinal (duodenal) biopsy is considered the “gold standard” for diagnosis because it will tell you (1) if you have celiac disease, (2) if your symptoms improve on a gluten-free diet due to a placebo effect (you feel better because you think you should) or (3) if you have a different gastrointestinal disorder or sensitivity which responds to change in your diet.
If the results of the antibody or genetic screening tests are positive, your doctor may suggest an endoscopic biopsy of your small intestine. An endoscopy is a procedure that allows your physician to see what is going on inside your GI tract. A scope is inserted through the mouth and down the esophagus, stomach and small intestine, giving the physician a clear view and the option of taking a sample of the tissue.
This is usually an outpatient procedure. Samples of the lining of the small intestine will be studied under a microscope to look for damage and inflammation due to celiac disease. It is recommended that the doctor take at least 4-6 duodenal samples from the second part of duodenum and the duodenal bulb, in order to obtain an accurate diagnosis.
Source: Celiac Disease Foundation