Food allergy testing
Skin Scratch Testing is the cornerstone in allergy diagnosis, it has been used for over 100 years. Skin testing is cheap, safe, easy to do and someone in the practice can easily be trained to perform it. Commercial inhalant allergens are readily available, but food allergens are a little more difficult to come by. The main problem with food allergens is the lack of stability of extracts. A few food allergens such as Peanut, Egg, Wheat, Soya, Tree nuts, Fish and Cocoa are stable and commercial allergens are available. However, fruit and vegetable allergens are very unstable and rapidly denature rendering commercial extracts unreliable. Infants can be skin tested for food allergy from 4 months of age.For this reason, for fruit and vegetable allergens, we tend to use the Prick plus Prick test, where we prick the offending food, gather some sap and then prick the patient with the fresh extract. This form of testing is highly reliable and possibly more accurate than RAST tests.
When Skin or Prick plus Prick test are not available, RAST tests should be used - Many of the RAST fruit allergens extracts are labile and only have a 60% accuracy. They can also be very expensive and blood has to be sent to specialised immunology laboratories to be processed. The negative predictive value of food allergy testing is good - if a test is negative, then there is a 90% chance of there being no allergy to that food., but the positive predictive value is less specific - a positive test indicates sensitisation to that food but does not necessarily confirm a clinical allergic response will occur.
The Double blind placebo controlled food challenge test is the gold standard for food allergy diagnosis, but is totally impractical in the GP setting as it can be very dangerous and trigger anaphylaxis. Only specialised allergy centres with in-house allergy dieticians and full resuscitation equipment are prepared to do this form of testing. Open food challenges (non-blinded) with the suspected food allergen is the more popular procedure in most clinics. Traces of the offending food are introduced to the person, initially by rubbing onto the skin. Then contact is made with the eye or lip. Next traces are placed on the oral mucous membranes, until finally a substantial amount of the food is consumed without reaction. These challenges take place under close medical supervision at 15-minute intervals. The test is aborted if an adverse reaction occurs at any stage.
Conventional Blood Tests for Allergy
(RASTs and UniCAP)
The newer RAST tests are called UniCap RAST tests and measure specific IgE in the serum to different food allergens. To simplify matters, there are various screening panels.In children the common food allergens are contained in the fx5 Paediatric Food Screen, and these are Cow’s milk, Hen’s egg white, Wheat, Soya, Peanut and Codfish. There is a Tree Nut Screen called fx1, which contains Brazil nut, Hazelnut, Almond, Coconut and Peanut. Then there is a seafood mix called fx2 that contains Codfish, Shrimp, Blue mussel, Tuna, and Salmon. The cereal mix screen called fx3 contains Wheat, Oat, Maize, Sesame seed and Buckwheat. Another useful food screen is the fx7, which mix contains Tomato, Yeast, Garlic, Onion and Celery.
There are up to 160 different food allergens that can be used to test for specific IgE antibodies, and range from Almond through to Youngberry.
Some foods such as milk and certain fruits contain a large number of different potential allergens and one has to be aware that a specific RAST may only test a few of the potential allergens in the food. Some people who are read more
