Before diagnosing a food allergy, a physician will take a medical history and perform a physical examination of the patient. The medical history also can provide crucial clues to the nature of a food allergy or intolerance. The physician is likely to ask about several aspects of the allergy, including the nature of the reactions—when they occur, how quickly they come on, and whether they are always associated with the same foods.

Sometimes, it is possible to make a diagnosis based solely on the patient history. In such instances, a patient may be asked to keep a diary of each meal, including foods consumed and any reaction to them.

An elimination diet is the next likely step. The physician asks the patient not to consume certain foods or ingredients for a few weeks, while suggesting alternative sources of any lost nutrients. For example, patients who eliminate eggs receive suggestions for other sources of protein. It is also best if patients can eliminate all other possible sources of an allergy, such as medications, vitamin pills and exposure to any other triggers known to cause allergies in the individual being tested.

During the testing, the patient must be very careful not to consume any amount of food that contains the restricted allergen. The patient will have to carefully read all food labels and inquire about food preparation methods when dining out.

The elimination diet often involves two stages. The first stage excludes the most common food allergens (e.g., milk, eggs, wheat, soy, corn). The second stage is eliminating many foods except those that, as directed by the physician, are least likely to create an allergic reaction. The foods permitted in this stage will vary from person to person.

In rare instances, a physician may recommend fasting as a method of detecting food allergies. Although this is an extremely effective method for identifying food allergens, it is usually reserved for people who are suspected of having multiple food allergies.

If the patient’s symptoms disappear during the period when foods are restricted, a food allergy or intolerance diagnosis is likely. If a food allergy is suspected, foods usually are reintroduced, one-by-one, at intervals of three to seven days. Suspected foods are typically eliminated for about three weeks before being reintroduced in the following sequence:

• Fruits and vegetables
• Poultry, seafood, eggs, red meat and dairy products
• Cocoa, sugars, nuts and grains

Each time a food is reintroduced, the patient keeps a record of every food consumed, and any symptoms that might reappear.

If symptoms do return, it adds further weight to suspicions that a food allergy is present. The food is eliminated from the patient’s diet one more time. If symptoms disappear once again, the allergy diagnosis is confirmed.

Some physicians may also want to confirm a diagnosis by performing a traditional allergy test, such as:

• Allergy skin tests. A small amount of an allergen is introduced to the patient’s skin to determine if the person is allergic to that allergen.
  
  

  
  

• Blood tests. A sample of the patient’s blood is tested for substances that indicate an allergic reaction has occurred to a specific allergen. These tests can sometimes detect substances that indicate food intolerance.