The first step a physician is likely to take in the diagnosis of a food allergy is to create a detailed medical history and dietary history of the patient. The history may include the following information:

• Timing of the reaction (e.g., whether the reaction occurred within an hour of eating)
  
  
• Whether the reaction is always associated with a certain food or food group
  
  
• How much of the food was consumed, because the severity of a reaction may relate to the amount of food consumed
  
  
• How the food was prepared (e.g., was it consumed raw or thoroughly cooked)
  
  
• Were symptom relief measures successful (e.g., if taking antihistamines relieved the symptoms)

The physician will also perform a physical examination to help identify or exclude medical problems that may be causing the patient's symptoms. By gathering this basic information, a physician can get a better idea of where to proceed with further evaluations. This may include attempts to identify the specific allergen to which the patient is sensitive. Such procedures are not definitive, but can provide information that is relevant to the patient’s condition and useful in designing treatment plans. These measures may include:

• Skin test. This test involves pricking the patient’s skin with food extracts. The skin will react with redness and swelling for those extracts capable of eliciting an allergic reaction when consumed. This accuracy of this test's results will vary depending on the food being tested.
  
  

  

• RAST (radioallergosorbent test). This type of blood test allows a laboratory to directly test a blood sample for antibodies that correspond to specific foods. While less sensitive than a skin test, it can be used on individuals who have reactions too severe for a skin test. The RAST test is also a good option for infants, or individuals with skin disorders such as eczema.
  
  
• Oral food challenge. Also called the double-blind food challenge, this is considered the most effective way of determining the cause of a food allergy because it supplies the most convincing results. Different foods are placed within capsules to hide their identity. Neither the patient nor the physician knows which capsule contains the suspected allergen. The patient consumes the capsules and the physician looks for signs of an allergic reaction. This type of test is time-consuming and difficult. It is often reserved to confirm suspicions that a patient's symptoms are not caused by a food allergy. This type of test should only be performed in the presence of a physician who can treat anaphylaxis.
  
  
• Elimination diet. This process involves removing suspect foods from a patient’s diet to see if the allergic reactions persist. Foods are removed on a trial-and-error basis for a period of time. The results of the diet make it easier to confirm, rule out or permanently remove a problem food.
  
  People should not attempt to “self-diagnosis” their food allergy by eliminating foods on their own. Performing an elimination diet without the supervision of a physician can lead to an unbalanced diet and a number of resulting health problems.

Physicians may also ask their patients to keep a food diary over the period of a week or two. While keeping the diary, patients will note all of the foods they eat, the symptoms they experience and when the symptoms occur. This information can be used in combination with findings from the medical history, physical exam and allergy tests to determine the particular food that is triggering symptoms.