Skin Scratch Testing is the cornerstone in allergy diagnosis; it has been used for over 100 years. Skin testing is cheap, safe, and 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
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. Cocoa
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 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.
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