A careful history and observation of food-related reactions are most important.
The food history should include:
- Identification of suspect food
- What is the time lag between eating and development of symptoms?
- What types of symptoms ensue?
- What amount of food is required to cause reaction?
- Does the reaction occur with every ingestion of the food?
- Does the reaction only occur under certain circumstances, e.g. exercise or stress?
- When did the last reaction occur?
- Are there other known allergies, e.g. hay fever, asthma, eczema?
- Do other family members have asthma?
The following tests can be helpful in determining sensitization (reaction of immune system), but are inaccurate in the diagnosis of food allergies.
Skin prick testing
A negative test excludes an allergy with a certainty of approximately 90% (sensitivity). However, a positive test confirms a specific allergy only in 50% of cases. The quality and standardization of tests differs widely and these tests only apply to IgE mediated allergies. The use of these tests together with a careful medical history can lead to accurate diagnosis in approximately 70% of patients. Strong reactions to skin testing may occur in highly sensitive individuals.
Serum-specific IgE blood levels can be measured, for example using the radio-allergo-sorbent-test (CAP RAST). However, IgE produced in the intestines in response to ingested food may not lead to raised blood IgE levels. Similar to the skin tests, a negative test excludes an allergy with a certainty of approximately 90% (sensitivity), but a positive test is more difficult to interpret. However, newer tests may allow a better predictability of positive results in egg, fish, milk protein, peanut and tree nut allergies. Threshold values are being defined for clinical use. IgE-independent measures, such as ECP and EPX in the blood or stool can be helpful in supporting a suspected diagnosis of food allergy.
Basophil activation tests (BAT)
These newer tests measure the expression of particular activation markers on specific cells involved in IgE-mediated allergies (basophils). They have been validated in various food allergies and complement the above conventional tests. The exciting promise of these tests, however, is that they can discriminate between sensitization (a laboratory phenomenon demonstrated by IgE and skin tests) and clinical allergy. Further testing is required to confirm the place of BAT tests in clinical practice.
Food challenge testing
A useful but cumbersome form of testing is the double-blind, placebo-controlled food challenge by a specialist. Increasing doses of the suspected food or a placebo are given at intervals and the patient is observed for signs of food allergy. As these challenges carry a small risk of severe reactions (anaphylaxis), they should be performed under adequate medical supervision. Food challenge testing does not reliably distinguish between intolerances and allergies and tests are not well standardized. Open food provocation tests carry similar risks, but are less conclusive.
Sequential elimination of specific food groups from the diet may allow identification of the offending food. Standardized elimination diets are available, with stepwise introduction of new foods every 2-3 days and documentation of food reactions and symptoms using a food diary. Such diets are best performed under the guidance of an experienced dietician.
New endoscopic allergy testing is being pioneered, but is not validated enough for clinical use. If the above tests do not lead to a clear diagnosis, exclusion of other diseases is warranted.