Winter 2011 Edition
Interpretation of allergy tests
by Raphael Strauss, MD, FAAAI, FACAAI
Q: Why do numbers double and triple in RAST testing over the years if the patient is not exposed to the allergic food?
First, let's discuss what RAST testing is and the significance of the test. Blood allergy tests, referred to as “RAST” testing measures specific allergic antibodies called IgE. In the case of food allergy, the diagnosis of food allergy is not based on any specific test; the diagnosis is based on the clinical history with supportive evidence on a skin test or blood test (RAST). There are patients who have minimal responses on allergy tests and severe allergic reactions and patients who have very high numbers and are NOT allergic to the specific foods. False positive and false negative or inaccurate tests are possible and the assistance of an allergy specialist in interpreting these results is important.
Once the diagnosis of food allergy to a specific food has been established, most allergists will recommend periodically retesting the patient. There is scientific evidence that supports the theory that if the size of the skin test or the numbers on the RAST test drop significantly, the patient may no longer be allergic or may be “outgrowing” the allergy. The diagnostic test that is most important is a food challenge. If the numbers on the RAST test or the size of the skin test and the clinical history indicate there is a reasonable chance that the patient may not be allergic, the allergist may recommend a food challenge. This means that the patient brings the food in to a hospital, clinic or office where personnel who are expert in the treatment of allergic reactions are available. The patient then ingests increasing amounts of the food under careful observation and if no reaction occurs, the patient can be considered not allergic to that food.
In periodic testing, RAST test numbers may go up or down. Although there is evidence that a significant reduction in numbers may indicate a favorable response on a challenge, there is little scientific support for the concept that higher numbers mean that the allergy is “getting worse.” The numbers may vary considerably with the type of assay that is run and the lab in which it is performed. Additionally, there are other immunologic factors at work such as protective antibodies and suppressor immunologic white blood cells that may be changing the immune response. In lay terms, it is kind of like looking at the score card and seeing who scored the most points. It does not tell who played great defense and turned the tide of the game!
Bottom line: I interpret allergy test results in the context of the individual patient history. Although I think it is great when the numbers drop, I am not particularly concerned when they rise, although it does suggest that the patient is not outgrowing the allergy.