These tests show the presence of the IgE antibodies. If these tests are negative an immediate type allergy is almost always ruled out.
Because the tests show the allergy antibody, they will start to be reactive (more than zero) when the person is sensitised (see above). A test that is “reactive” does not automatically mean that the person is allergic, it means the person has the potential to be allergic!
The larger / higher the reaction is, the more likely it is that the person is truly allergic to that allergen rather than just being sensitised. The results of the test must be carefully compared with levels that have been shown to be truly positive, rather than just a little raised (reactive).
For breathed in (aero-) allergens, a level of 3mm or more is usually considered definitely positive. For food allergens the “positive” values vary between foods and at different ages, but are usually much higher than for aero-allergens.
In some cases the test cannot give the final answer and other tests may need to be done. Allergy tests are a guide to whether the person has the potential to be allergic but they do not predict whether the reaction will be mild or severe.