Diagnostic procedures for food allergy
A correct diagnosis of food allergy is essential for proper treatment of the allergy symptoms in infancy and childhood. Unproven allergy tests should be avoided.

Allergy experts continuously review all types of allergy tests currently available. Allergy experts support several tests including: (1) Double blind oral challenge test. In this test both the physician and the patient are not told of the challenge food presented usually in a capsule, (2) Blood test for the total and food-specific IgE. The CAP RAST test is a convenient and currently the preferred test, (3) Skin prick test. In this test the allergens are introduced subcutaneously and a reaction is observed few minutes later, and (4) The atopy patch test. In this test the suspected food allergen is placed in close contact with the skin for a period of 48 hours and sometimes longer. The development of an eczematous skin at the site of contact suggests a positive reaction.

 

Firstly, the "gold standard" of food allergy diagnostic testing is a “double-blind, placebo-controlled oral food challenge.” When performing this test, the physician should have a clear idea of the patient’s allergic condition and be ready to treat anaphylaxis. Informed consent from the patient or the patient's guardian is also required. A negative response to a double-blind, placebo-controlled challenge must be followed with an open oral food challenge performed under controlled conditions to ensure that the food can be tolerated safely. Since the test is clinically time-consuming other tests are often performed before considering the oral food-challenge test.

Secondly, measurement of the food specific IgE level using the CAP RAST remains the “in-vitro gold standard.” Certain level of response allows for reasonable prediction of clinical relevance. The results from the CAP RAST allows for meaningful approach for the treatment of the allergy problem.

There are other in vitro blood test methods available but without appropriate in-built standardized controls they may give variable results under different laboratory conditions. Such inaccurate results may confuse the management of the allergy problem. Such imprecise and unproven blood tests should be avoided. Several studies show that some of these tests are either over sensitive (give false positive results) or under sensitive (give false negative results).

Thirdly, the skin prick test is a valid approach when conducted with care particularly after taking into consideration the medical history of the patient. Unfortunately, the skin prick test does not provide independent information on the total IgE level which is an useful marker for the seriousness of the allergic condition. The IgE level which can be obtained through blood test is a useful bench mark of the allergy status of the patient for future reference. Moreover, the skin prick test can only be carried out after taking several parameters into consideration.

Fourthly, the atopy patch test may be helpful in the diagnosis of food allergy in about 25% of infants with food allergy. When the patient has the IgE-mediate immediate reaction and the late reaction to foods at the same time, then this test will distinguish the late reaction which may be missed if only the IgE-mediated test is determined.