Another test that requires a needle is the RAST, a test that measures the amount of allergen-specific IgE in your blood. This test doesn’t require an allergist; your GP can perform the test, but an allergist may be more qualified to interpret the results and is usually the doctor who performs the test. RAST consists of drawing a small amount of blood and then having the blood tested — by sending it out to a lab. Doctors can perform RASTs for almost any food or airborne allergen.

The most important point about RASTs is that they’re not all the same. Some types of RASTs are more accurate than others, and the results of one type of RAST are not interchangeable with the results of another type. For diagnosing food allergies, the type of RAST that has the best track record is the Pharmacia CAP fluorescent enzyme immunoassay. Wrap your mouth around that one! To simplify the nomenclature, doctors refer to this type of RAST as CAP-FEIA or CAP-RAST.

As with skin testing, negative RAST results are quite accurate in ruling out an IgE-mediated food allergy, but positive RAST results do not necessarily mean you have a true food allergy. False positive results occur with RASTs for the very same reasons they occur with skin testing. However, because the RAST is more of a true measure of the amount of IgE in your system, differentiating a true positive test from a false positive test is generally easier than it is in the case of skin tests.

When your doctor gets the results, she looks at your RAST score and interprets the results based on the following criteria:

1. The higher the RAST score, the more likely that the results represent a true food allergy.

2. More importantly, for some of the most common food allergens, the doctor may compare your RAST levels to predetermined cut-offs, above which a true food allergy is almost certain. For example, using the CAPRAST, which gives results on a scale from 0 (zero) to 100, an IgE level of more than 7 to egg, over 15 to milk, 14 to peanut, and 20 to codfish is highly predictive (greater than a 95 percent chance) that you’re allergic to that food.

Your doctor can often use RAST results to track your levels of specific IgE antibodies over time. RAST levels that decrease over time are an excellent indication that you’re outgrowing your allergy to a particular food. My colleagues and I typically decide when to try to re-introduce a food into a patient’s diet based on the RAST result.

Related posts:

1. Food Allergies Diagnosis, Interpreting skin tests
2. Food Allergies Diagnosis
3. Food Allergies Diagnosis, General Practitioner
4. Food Allergies : Navigating the diagnostic process
5. Food Allergies- Unraveling the Truth Through Blood Tests