Blood tests measure the presence of IgE antibodies to specific foods. (IgE, short for “immunoglobulin E,” is the antibody that triggers food allergy symptoms.) Known as RASTs (which stands for radioallergosorbent tests), blood tests may be performed alone or in combination with other diagnostic tests. For example, a RAST may be the best choice for a patient who has a chronic skin condition or who is taking antihistamines, which would interfere with a skin prick test. Allergists also use blood tests to monitor patients who are already known to have a food allergy. In this case, the test may help determine if a person is outgrowing the allergy.
In this test, a sample of your blood is sent to a laboratory, which measures the level of IgE antibodies in the sample. The results take about a week to arrive and are based on a numerical scoring system. Not all RASTs are the same, and different laboratories sometimes use different “brands” of the test. In addition, different tests may use different scoring systems. In general, however, the higher your score, the more likely you are to be allergic to the food being tested.
A test known as the CAP-RAST or CAP-FEIA (brand name: ImmunoCAP) is a type of RAST that can measure exactly how much IgE is present to a particular food allergen. The CAP- RAST reports IgE levels on a scale from less than 0.35 (undetectable) to more than 100 in a measurement called “kUA/L.” Over time, researchers have pinpointed scores that indicate, with good accuracy (above 95 percent), that a patient is allergic to specific foods. For example, if a young child has a CAP-RAST level of more than 7 kUA/L to egg, 14 kUA/L to peanut, or 15 kUA/L to milk, it is very likely that she is allergic to that food.
Over a period of years, an allergist can monitor CAP-RAST scores to help determine whether or not patients are outgrowing their allergies. For instance, most children eventually outgrow their egg or milk allergy. When the test shows that the IgE level is low enough, the allergist can carefully supervise the reintroduction of important nutrients into a patient’s diet.
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:
- When you eat, your digestive system gradually breaks down food proteins into very small pieces. As a result, the allergenic proteins may be so small that the IgE antibodies are unable to detect them, so the food is actually safe for you to eat. But blood and skin prick tests can’t mimic the digestive process. Since food proteins are bigger when they interact with your blood or skin, it is easier for the IgE antibodies to “see” the allergens and attack them. Your tests, therefore, may show that you are more sensitive to a suspect food than you really are.
- Members of a food “family” often share similar proteins. 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. This is known as cross-reactivity. The test is positive because it recognizes a similar protein in peanuts and green beans. But the test hasn’t detected the real culprit – another, different protein that is found only in peanuts.
Despite the high rate of false positives, 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 skin prick 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.