food allergy testing

Diagnosing and treating food allergy appropriately presents one of the more difficult challenges for the practising allergist. There are no foolproof laboratory tests and inappropriate elimination diets may have serious nutritional consequences. It should also be noted that some food allergic reactions can be temporary, particularly in young children.

It is important to remember that adverse reactions to foods are not always allergic in nature. As an example, lactose intolerance is the result of a deficiency of the enzyme lactase and is not an allergy. Similarly, toxic reactions may occur in cases of food poisoning. These reactions do not represent allergic reactions.

Taking a careful history of the reaction is the cornerstone in establishing the diagnosis. If there is a repeated and clear relationship between the consumption of a specific food and the reaction, the diagnosis of food allergy is likely. Similarly, the more severe the reaction the more likely that a specific food allergen will be identified.

IgE antibodies are involved in many allergic reactions to foods. IgE antibodies can be measured by either skin testing or by RAST testing of serum in the laboratory. Each method of testing has its advantages and disadvantages. A summary is presented below.

Advantages of RAST testing

RAST test results can be expressed either as an absolute value (International Units) or as a class (1-4). 4+ reactions are the most intense. RAST tests may be superior to skin tests in some situations:

• Antihistamines may interfere with skin tests hence RAST testing may be advantageous where antihistamines cannot be discontinued for medical reasons.
• Similarly, patients with extensive skin disease and those with dermatographia (where a non-specific wheal and flare reaction occurs on contact with the skin) require RAST testing.
• Performance and interpretation of skin testing requires considerable skill. RAST tests can be useful in locations where skin testing is unavailable. Serum can be transported to laboratories performing the RAST tests.
• Serial RAST measurements may be useful in predicting when a child may be able to tolerate a food he or she reacted to previously, but it is crucial that this be determined by an allergy specialist or experienced doctor. Do not reintroduce foods without medical supervision.

Advantages of skin testing

Skin testing may be superior to RAST testing in some circumstances:

• In this country, the panel of skin tests is more extensive than that for RAST tests hence a RAST test may not be available for some foods (or some inhalant allergens).
• Skin tests may be advantageous in the identification of some food allergy reactions which require the use of fresh food (particularly fruits and vegetables) as the allergen may be lost during the preparation of commercial RAST disks.

Disadvantages of RAST tests

Both RAST tests and skin tests to foods can cause false positive reactions and may need to be interpreted with caution:

• Many patients with eczema have very high levels of total IgE. Low-level false positive results may be seen in these cases. This is because there is so much IgE present in the blood sample that it shows up as a positive result for foods that the person is not allergic to.
• Similarly, foods with similar protein structures may cross-react, resulting in false positive results. Generally, false positive reactions tend to show up as RAST classes 1 and 2.
• It is unusual for individuals to be allergic to more than 3-4 different food groups. If the patient has many positive RAST tests (especially RAST classes 1 and 2) to foods, these may represent false positive reactions. In contrast, a negative RAST or skin test (see below) reaction is more helpful in that allergy to the suspected food becomes much less likely.

Disadvantages of skin tests

Similarly, even when performed optimally, food allergy skin tests must be interpreted with caution.

• Some studies have shown that only 1/3 of positive food skin tests could be confirmed by double blind food challenge. Eliminating all foods to which the patient reacts to on skin testing may lead to nutritional problems.
• In contrast to these false positive reactions, false negatives are less common. As with RAST tests, a negative skin test reaction to a suspected food allergen is likely to be helpful in eliminating it as a cause of a food reaction
• Skin tests do not offer the same predictive information that RAST tests offer. Many food allergic children will continue to have positive skin tests even after they have stopped reacting clinically. The size of the wheal and flare may not correlate with the clinical severity of the reaction.
In contrast to food allergy testing, false positive respiratory allergen and insect venom RAST and skin tests reactions are less common.

The role of IgG antibodies

The relationship of IgG antibodies to food allergy remains to be established. Many asymptomatic individuals generate IgG antibodies to foods and other environmental substances. These IgG antibodies may simply represent exposure to foods in otherwise well individuals and may have no relevance to food allergy.

Short-term elimination diets

Short-term elimination diets (2-3 weeks) can be helpful in some patients. It is important that the food is totally eliminated as exposure to even small amounts of the food protein may lead to eczema. In the case of infants being breastfed, the mother may also need to eliminate the food from her diet. Some maternal food proteins have been shown to cross into breast milk. A dietitian may be helpful in designing a diagnostic elimination diet.

Food challenges

Food challenges can be performed either in an open protocol or by double blind challenge. The gold standard for food allergy diagnosis remains the double blind placebo controlled food challenge. These studies are undertaken in hospital where the patient receives a series of capsules or liquid containing either the food or placebo. The reaction to each capsule or liquid is documented before the code is broken. If there is any doubt about the diagnosis of food allergy, this should be undertaken before embarking on a long-term elimination diet.

Treatment of food allergy

At this time, elimination diets are the only effective treatment for proven food allergy. It is essential that the offending food be totally eliminated from the person's diet. Long-term elimination diets should not be undertaken without consulting a dietitian to ensure that the patient is on a nutritionally sound diet. A dietician may also be helpful in identifying hidden sources of foods the patient is allergic to.

A final note of caution

Laboratory errors do occur occasionally. Specimens may get mixed up and in spite of stringent controls, machines can malfunction. It is essential that the results of skin testing and RAST testing be correlated with the clinical picture. Specifically, food challenges must not be undertaken in an individual with known life-threatening reactions, based on a single RAST result. A discrepant result must be repeated and if necessary, the laboratory immunopathologist must be consulted for advice.

Written by Immunologist Associate Professor Rohan Ameratunga