The below tests are useful in the differential diagnosis of
food-related allergies, but they may quite frequently yield inaccurate results.
They are useful additions to a careful history and observation of food-related reactions.
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?
- Other allergies, e.g. hay fever, asthma, eczema
- Do other family members have asthma?
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.
Laboratory tests
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. IgE-independent
measures, such as ECP and EPX in the blood or stool can be helpful in supporting
a suspected diagnosis of food allergy.
Food challenge testing
A useful 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.
Elimination diets
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 (See
Food diary).
Such diets are best performed under the guidance of an experienced dietician.
If the above tests do not lead to a clear diagnosis, exclusion of other diseases
is warranted.