You may find it difficult to understand your allergy results. The following represent common questions asked of Dr Du Toit re Allergy Tests
Why do you only perform SPT and Specific-IgE allergy tests?
There are only two validated allergy tests for the diagnosis of immediate-onset food allergy; these are the Skin Prick Test and IgE-measurement (Dr Du Toit uses the Standardised Phadia IgE-System). There are NO other laboratory tests available (outside of research centres) that are sufficiently predictive of allergy to be used in ethical clinical practice.
Why are these tests performed in such different ways i.e. one is done on the skin and the other in the Lab on a blood sample?
Specific IgE is the allergy antibody. It is made in your bone marrow and then travels in low quantities in the blood (where it can be drawn for measurement) to deposit in high quantities in the tissues (where it can be measured by skin prick test). Indeed, the majority of IgE deposits in the tissues, and it is for this reason that skin tests are usually preferred and performed in the first instance. There are other distinct advantages and limitations peculiar to each test. What can make specific IgE testing e.g. peanut IgE, difficult to interpret is the reading of specific values in the presence of a high Total IgE. High Total IgE's are common in children of African or Asian descent and in children with severe eczema. High Total IgE's generate a lot of background 'noise' in the test making low values difficult to interpret. The analogy i use is listening to an orchestra, if you seek out a single instrument sound playing in a 15 piece Orchestra this is relatively easy, but doing so in a large Symphony Orchestra of 100 pieces is far more difficult as the sound gets drowned out (unless very loud). I personally place a greater emphasis on SPT'ing if done using a standardised technique and with fresh and reliable allergen extracts.
How are the tests reported and can these results be compared between tests and within tests but for different foods?
Skin Test Results are reported in mm (wheal and flare diameter, at widest diameter), tests may range from 0 - about 30mm, depending on the food; raw egg usually produces the most impressive results. Specific IgE testing is reported in kU/L, results can range from 0.01-100 KU/L. Test results - for the same food - cannot bedirectly compared using the different tests ie a SPT of 5mm to egg is not the same as 5 KU/L to egg using IgE-testing. In addition, specific food values cannot be compared with other foods e.g. a 5mm to egg carries a different certainty compared to 5mm to peanut.
Can the test size predict for severity of future reactions?
No, the size/level of the allergy Test result (in mm's or KU/L) does not predict for severity of future allergic reactions. Test size does however predict for the likelihood of a reaction occurring in the event of accidental exposure.
We are improving the ability to predict for future severe reactions using component testing. Component testing makes use of advanced recombinant allergen technology which is particularly useful for the understanding of the likely severity of accidental reactions, chance of outgrowing the allergy, and potential of the allergen to cross-react with other foods.
Can allergy tests predict if an allergy is being outgrown?
Yes, tests that rapidly diminish in size over time suggest that the allergy is being outgrown. Low test sizes at presentation also carry a good prognosis wrt outgrowing the allergy.