RAST vs. skin test info from Dr. Sicherer

This is straight from a handout we received at the spring FAN conference in Chicago. The speaker was Scott Sicherer, Assistant Professor of Pediatrics (Allergy/Immunology) at Mt. Sinai.

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Since the majority of illnesses caused by food allergy are IgE-mediated, specific tests can help to identify or eliminate from consideration, responsible foods. One simple method to determine the presence of IgE-mediated antibody directed toward food proteins is prick-puncture skin tests. THis is a quick, inexpensive method to detect IgE antibodies to partocilar foods and can be done at any age. A standard device, such as a bifurcated needle, plastic probe or lancet, is used to puncture the skin through a liquid extract of the food. An immediate mosquito-bite looking reaction at the site of the skin test indicate the presence of food-specific IgE antibly. You must not be using any antihistamine medications for these test to be accurate. Injecting food extracts into the skin with a needle (intradermal) for testing purposes gives an unacceptably high false positive rate, is dangerous and should not be used. The sizes of the bump (wheal) and redness (erythema) of the food tests are compared with the "control" tests.

Another way to test for food-specific IgE antibodies is with a blood test called the RAST test. Your doctor should be certain that the test being ordered is for IgE antibody to the food(s) in question. In most cases, the skin tests are better at detecting small amounts of IgE than these blood tests, but the blood tests offer some advantages. The bllod test can be used even if you are taking an antihistamine or when severe rashes preclude finding a spot to place skin tests. These tests are more expensive than the skin test and you will have to wait for the results to be reported from the laboratory. Test results are reported in a variety of ways depending on the lab and particular test used. THe most common reported units are "classes" usually on a scale of 0 to 5 or 6.

Negative tests virtually exclude IgE-mediated allergy to the food tested, but a positivie test does not prove symptomatic allergy.
THIS IS WHERE THE DIFFICULTIES AND MISUNDERSTANDINGS ABOUT ALLERGY TESTS ARISE!!! [his caps] A negative skin prick test is very good (correct over 19/20 times) at excluding an IgE-mediated reaction to the particular food tested. Unfortunately, when a test is positive, it does not mean that the person would definitely experience a reaction to the food tested. In fact, the chance that the person is symptomatically allergic in the face of a positive skin test is on the order of only 50% (like a coin toss). Thus, a positive skin test alone cannot be considered proof of symptomatic allergy. This is why it is so important to evaluate the tests in the context of the medical history of reactions. It is also why it is not a good idea to randomly test huge numbers of foods, since many of the positive tests would be irrelevant.
Like the skin tests, a negative RAST test usually eliminates the possiblity of an IgE-mediated reaction from the food tested, but a positive test does not always indicate symptomatic allergy. The level of specific IgE antibody measured using a particular method of RAST test (CAP-RAST FEIA reported in units called KU[subA]/L) was reported to be useful in determining the chance of true reactivity to certain foods in children with allergic skin rashes. For example, and IgE antibody level of over 6KU[sub A}/L to egg, over 32 to milk, over 15 to peanut and over 20 to codfish were highly predictive (95 out of 100 chance) of having some type of allergic reaction. Unfortunately, lower values, unless virtually undetectable, may still indicate an potential for having an allergic reactions. THAT MEANS THE TEST MAY BE POSITIVE BUT UNLESS IT IS PARTICULARLY HIGH, IT MAY NOT REFLECT A TRUE POTENTIAL FOR A REACTION. [again, his caps] Also cut-off numbers are not determined, as yet, for any other foods. This test may also prove useful in following levels of particular IgE antibodies over time to see if they are falling (perhaps indicating that the allergy is being outgrown).

The interpretation of these test is complicated and your doctor must take into consideration your complete medical history, particularly the history of reaction, in applying the test results to your individual sitiation. UNFORTUNATELY, THE SIZE OF A SKIN TEST OR THE LEVEL OF A FOOD-SPECIFIC IGE ANTIBODY ON RAST DETERMINED FROM A BLOOD TEST DOES NOT SPECIFICALLY INDICATE THE SEVERITY OR TYPE OF A REACTION THAT WOULD OCCUR WITH INGESTION OF THE FOOD IN QUESTION. [his caps] Thus, a person who is "RAST class 5, skin test 4+" to peanut may have a milde reaction than someone who is "RAST class 2, skin test 2+." In fact, these test results might be found in a person who eats peanuts without any reaction! Furthermore, after a clearly allergic individual has outgrown an allergy, their test may remain positive for years. To further complicate matters, some of the tests may be positivie because there are proteins that "look similar" to the immune system but there may not be symptomatic allergy caused by these "cross-reacting" proteins. For example, most people with peanut allergy will also have a positive test to soybean (another legume) but most, although not all, peanut allergic individuals can eat soy without a problem Grass allergic individuals may have positive tests to grains for similar reasons and the list goes on and on. As you can see, this all gets very complicated and your doctor must keep all of these possibilities in mind when deciding upon yoiur diagnosis. Another thing to remember is that no test is perfect and if unexpected results are seen, it is important to repeat the test. Creating more specific and predictive tests is an area being aggressively researched.

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Excuse the typos. [img]http://uumor.pair.com/nutalle2/peanutallergy/smile.gif[/img]