Blood tests measure the presence of IgE antibodies to specific foods. (IgE, short for “immunoglobulin E,” is the antibody that triggers food allergy symptoms.) In the past, these tests were called “RASTs” (which stands for radioallergosorbent tests) because they used radioactivity, but modern tests do not. Both the blood test and the skin prick tests detect food-specific IgE. With the skin tests, the result is immediate, but the blood test result will take at least several days to arrive. Unlike the skin prick test, the blood test is not affected by antihistamines and can be performed for people with extensive rashes that prevent using skin tests.
Different laboratories sometimes use different “brands” of the blood test and may report results using different scoring systems or units. Your allergist must be aware of these differences because the tests and reporting systems are not interchangeable.
Your allergist should explain the meaning of the blood tests to you. The results are not very helpful for predicting the severity of an allergy. Instead, the test gives information about the chance that there is an allergy. This test is not like a pregnancy test, in which a person is or is not pregnant. Often, the blood test is positive, but the person is able to eat the food without symptoms – they are not allergic. There are a number of possible reasons the test may be “positive” but the person is not truly allergic:
About 50-60 percent of all blood tests and skin prick tests will yield a “false positive” result. This means that the test shows positive even though you are not really allergic to the food being tested. These results occur for two reasons:
- The test may be measuring your response to the undigested food proteins. It is possible that after digestion, the food protein that enters your bloodstream is no longer detected by your IgE.
- The test may be detecting proteins that are similar among foods but do not trigger allergic reactions. For example, if you are allergic to peanuts, your tests may show a positive response to other members of the legume family, such as green beans, even if eating green beans has never been a problem for you.
Despite the high chance that the blood test or skin test may be positive despite there being no allergy, or, rarely, negative despite there being allergy, in the hands of an experienced allergist, blood and skin tests are extremely helpful. This is especially true when the results are interpreted in the context of your medical history. For example, if your history suggests that you have had several reactions after eating soy products, and the blood tests show a positive reaction to soy proteins, it is very likely that you do have a soy allergy. Your allergist may order additional tests, if necessary.