Skin Prick Tests and specific IgE tests

These tests are first-line diagnostic tests which are suitable for patients of all ages. The skin prick test involves pricking the skin through a drop of food extract reagent and any resultant wheal measured against positive and negative controls. A result ≥3 mm than the negative control is considered positive although some recommend the use of a cut-off of 2 mm in infants or young children. Specific IgE antibody tests quantify the level of circulating antibodies to a specific food. They are may be the only test available in primary care but are also used in secondary care especially for individuals with severe eczema, dermographism or who cannot discontinue antihistamines. Both tests have a good negative predictive value, but their positive predictive value can be as low as 50% and underlines the importance of only testing for suspected foods identified from a convincing clinical history.  For both tests there are studies demonstrating the predictive value of the test based on the size of the wheal diameter or level of specific IgE for individual foods. However, there is great variability between studies, which have mainly been undertaken in children and only the common foods have been studied. There are two major issues with both tests; firstly, they often lack the heat-labile allergens in plant foods, such as PR10 or profilins, which can cause reactions in older children and adults. Some nut reagents have been enhanced with added PR10 allergens but not those for fruits or vegetables. For this reason, a prick to prick skin prick test using fresh raw fruits, vegetables, and soy milk might be helpful where these foods are indicated as triggers in the reported reaction. foods. The second issue concerns sensitisation to aeroallergens such as pollens and house dust mite which can then lead to spurious positive tests to a whole host of other foods. Allergens in tree nuts and peanuts share very great similarities with those in tree, grass and weed pollen, and sensitisation to house dust mite can also lead to irrelevant positive tests to shellfish for the same reason. Thus, detection of sensitisation alone is insufficient proof of clinical relevance without a supporting clinical history, so it is important not to undertake multiple tests to foods, especially to those which are currently eaten and tolerated

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