If you find, for example, that every time you drink milk or eat peanuts, you break out with hives, you might strongly suspect an allergy to these foods. And if you have a personal or family history of atopic eczema, hay fever, asthma, or hive reactions to nonfood allergens, then the possibility of food allergies is more likely. On the other hand, because the signs and symptoms of certain skin, gastrointestinal, and respiratory diseases may mimic those of food allergies, such conditions must be excluded by means of a careful medical history and physical examination before a food allergy is diagnosed.

After a preliminary evaluation has been made and it has been determined that foods may be the culprits, a number of diagnostic tests are available to help cinch the diagnosis. Scratch tests involve making a series of small scratches on the skin and placing minute amounts of test food allergens on them to see whether an allergy is provoked. Redness and swelling at the test site indicate allergy. In general, tests for eggs, nuts, fish, milk, peanuts, and wheat correlate fairly well with allergic symptoms.

Unfortunately, this is not the case with many other foods. In other words, a skin test may indicate that you are allergic to a specific food. But if you experience no symptoms when you eat that food, you are not allergic to it. Doctors refer to this as a false positive result, and as many as 30 percent of normal individuals may have false positive skin test reactions to various foods. On the other hand, a negative skin test result that is, one where a food allergen does not provoke a reaction is a far more reliable finding for excluding a particular food as a possible allergic culprit.

Intradermal testing is another form of skin testing; however, it is seldom used nowadays in food testing. In this type of skin test, the suspected material is injected directly into the skin, and the test sites are observed for redness and swelling. The test is no more useful than conventional scratch testing, but because of a real risk of provoking a severe, life-threatening allergic reaction, it is no longer recommended.

Although less sensitive than skin testing for discovering allergies, RAST, a relatively expensive blood test, may be used to determine the presence of IgE antibodies to specific food allergens. The major advantage of this test is its safety, which is a clear benefit when individuals who have experienced potentially life-threatening reactions to foods are being tested.

Food challenges, in which the offending substances are given to the patient under strict medical supervision (usually in a hospital), are the only truly definitive tests for food allergies. Challenges are performed in one of three ways: open challenges, single-blind challenges, and double-blind challenges. In open challenges, both the patient and the physician are aware which allergen is being given. Because this test is more subjective, it is the least exacting of the three methods. In a single-blind test, only the patient knows what he or she is getting, making the test somewhat more objective. In a double-blind test, the most objective of the three, suspected food allergens and placebos are placed in gelatin capsules, and neither the patient nor the doctor knows which is being given. Allergy symptoms provoked in the double-blind setting can be taken as firm evidence of a true adverse reaction to a food. None of these tests needs to be performed if there is a clear-cut history of allergic symptoms that can be related to a specific food or foods. In other words, if you are certain that your respiratory symptoms or the swelling of your lips, tongue, or face began shortly after eating a certain food, you would generally not need these tests.

Elimination diets have also been used for many years in the diagnosis of food allergies. As the term suggests, these tests consist of eliminating potential food troublemakers from the diet entirely and then reintroducing them one at a time to see if they cause symptoms. Elimination diets usually exclude the following foods from the diet: cinnamon, chocolate (including colas), citrus fruits (grapefruit, lemons, and oranges), corn (including cereals, flour, meal, oil, starch, and syrup), eggs, food coloring and preservatives, grains (barley, millet, oats, rice, and wheat), legumes (beans, peas, peanuts, and soybeans), and tomatoes. More restrictive elimination diets permit only applesauce, bananas, lamb, and rice to be eaten.

If symptoms persist on a strict elimination diet, it is unlikely that the restricted foods are the problem. But if they disappear when a particular food is eliminated and then return when it is reintroduced into the diet, the cause-and-effect link is strengthened. Although this form of testing is frequently tedious and time-consuming, it has the advantage of greater safety when compared to some of the other methods already described.

Two tests deserve special mention because they have proven to be entirely useless and nothing more than a waste of time and money. The first is called cytotoxic leukocyte testing. According to the theory behind this test, food allergens possess the ability to lower your natural white blood cell (leukocyte) count or to induce derangements in their functioning. Proponents of the test believe that such abnormalities can be determined by either giving an individual the suspected food allergen and then drawing a blood sample to look for the changes, or adding the food allergen directly to a test tube sample of the patient's blood to look for the effects.

Sublingual testing is likewise of no proven value. In this test, droplets of increasing concentrations of food extracts are placed under the tongue (sublingually) until a dosage that provokes allergic symptoms is finally reached. Once this information is obtained, various concentrations of the food allergens are then continually administered sublingually, usually several times a week, in hopes of "neutralizing" the allergy. Neither cytotoxic leucocyte testing nor sublingual testing has demonstrated usefulness for either diagnosis or therapy, and both have been abandoned by most reputable physicians.