The traditional approach to diagnosing celiac disease is a three-step process:
- Perform a biopsy of the lining of the small intestine. This is a surprisingly easy procedure which takes only a few minutes, although small children are usually sedated first, which adds to the cost and complexity of the biopsy. If the villi are damaged (flattened or atrophied mucosa), go to step 2.
- Place the patient on a gluten-free diet for six months or longer and then perform another biopsy. If the villi are healed, go to step 3.
- Put gluten back in the diet for six months or longer, and then perform a third biopsy. If the villi are again damaged, then the diagnosis is complete. At this point, the patient goes on a gluten-free diet for life.
All of the laboratory tests that can be performed are strongly affected by a gluten-free diet. Tests will return negatives if the individual has been on a gluten-free diet for some time, and there is much debate about the length of time a patient must return to a gluten-laden diet before being tested. It probably depends on many factors: the level of damage that was done before starting a gluten-free diet, the length of time the person has been gluten-free, the amount of healing that has occurred, and the sensitivity of the individual to gluten.
A tentative diagnosis of celiac sprue is usually offered if the patients symptoms clear up after some time on a gluten-free diet. This is often followed by a "challenge" in which one of the offending grains (usually wheat) is eaten to see if the symptoms reoccur. However, this approach is much less desirable than having a firm diagnosis from a combination of antibody tests and one or more biopsies.
Because a gluten-free diet precludes accurate testing, if you suspect celiac disease, it is advisable to have diagnostic tests performed before starting a gluten-free diet.
Some physicians will accept positive antibody tests, one biopsy, and improvement on a gluten-free diet as sufficient for diagnosing celiac disease. Many other doctors prefer to perform a second biopsy, feeling that if it shows normal villi after a period of eating gluten-free then the diagnosis is confirmed. There are still some doctors who prefer the three-step approach mentioned above, though most view this as unnecessary.