Not all adverse reactions to food are true allergies -- that is, IgE-mediated reactions in the immune system. Some are food intolerances or sensitivities. Because food intolerances can be triggered by a number of causes, making the diagnosis is somewhat difficult; tests for several common food intolerances are specific to those disorders. However, there are a few tests and techniques that you're likely to encounter when your doctor, allergist, or gastroenterologist is diagnosing food-related symptoms that don't appear to be true allergies.
If your doctor is not sure whether food is the cause of your symptoms, or if your doctor would simply like to look for patterns in what you've been eating when your symptoms occur, he may recommend that you keep a food log or food journal. This is a complete record of the foods you've eaten over a period of time. It's often combined with other diagnostic methods. Food logs may be used to narrow down potential migraine triggers or to assist training athletes in addition to their use in diagnosing food intolerances.
While different doctors prefer to conduct them in different ways, the basics of all elimination diets are similar. They consist of reducing your diet -- whether by cutting out the foods that are most likely to be causing problems or by reducing your diet to all but a few foods that are considered unlikely to cause intolerance symptoms -- and then slowly increasing the number of foods you eat while monitoring your symptoms. Elimination diets can be used to assess a variety of food intolerance symptoms.
While most diagnostic blood tests for food intolerances are considered controversial, blood testing is often used as an initial test for celiac disease (celiac sprue), a relatively common autoimmune disorder that causes the body to attack the small intestines upon eating a protein found in wheat, rye and barley. This test does carry some risk of false positives (that is, a result indicating celiac disease even though the patient doesn't have celiac disease) and false negatives (which are especially likely to occur in a celiac who isn't eating gluten-containing grains), and so it is usually followed by an endoscopy. Blood tests may also be used to screen for lactose intolerance.
An endoscopy is an examination and biopsy of the tissue of the small intestine, often used to test for celiac disease (as well as several other non-food-related conditions). In this test, a flexible tube with a camera attached is lowered into the stomach, usually through the esophagus. When checking for celiac disease, gastroenterologists look for patterns of damage in the villi, or small hairlike projections in the small intestine.
A test that measures the concentration of hydrogen in the breath is another method sometimes used to test for lactose intolerance. Hydrogen is a byproduct of lactose consumption in people with lactose intolerance. This test involves taking a baseline sample of the patient's breath, then taking samples over several hours after the patient consumes lactose to see if the concentration rises sharply.