How Do I Interpret the Numbers on My RAST Test Results?

A RAST test (RAST stands for "radioallergosorbent test") measures the level of allergen-specific IgE antibodies in your blood. In simpler terms, that means the test measures your immune system's reaction to a particular food allergen.

Even so, few doctors are using the RAST blood test to screen for allergies. Instead, they're using what's called the enzyme-linked immunosorbent assay, or ELISA, blood test, which is more sensitive and doesn't require the use of radioactivity.

However, your doctor may have reasons for preferring the older RAST technology. If that's the case, here's a guide to interpreting your test results.

Your immune system produces what are called antibodies to fight something it views as a threat, such as a virus or bacteria. When you have a food allergy, it means your immune system is making antibodies designed to fight against the food to which you're allergic. This is what causes allergic reactions.

When your doctor orders a RAST test, you'll have a blood sample drawn. The laboratory performing the test will then use the RAST technology to look for allergen-specific IgE antibodies in that blood sample.

However, this is where it gets a bit tricky. The RAST test can tell you how much of a specific antibody against a specific allergen is present in your blood. However, not everyone with the same concentration of antibodies in their blood will react in the same way to the actual presence of an allergen in their bodies (in medical terms, this is known as in vivo—literally, in the flesh).

In other words, someone with a relatively low concentration of one particular antibody in their blood may have a fairly severe reaction when actually encountering that allergen in the flesh. On the flip side, someone with a higher concentration of that specific antibody may not react at all, or just in a minor way, to the allergen when encountered in real life. In the second case, that person isn't truly allergic to the food, even though their blood test came out positive. (This is known as a false positive.)

RAST tests aren't very sensitive—they have a low rate of false-positive results and a fair number of false negative results, although this varies by the allergen. Overall, though, people who have lower amounts of allergen-specific antibodies in the RAST test generally tend to have a lower chance of reacting to the food in real life, especially if their skin prick test results also are negative, and if they don't have a history of reacting to that particular food.

Different foods have distinct levels that are considered "predictive" in the RAST test, meaning they can predict whether you'll have an allergic reaction to that food. Researchers determine these levels by comparing RAST test results to the results of blind food challenges in order to find a level of specific IgE on the blood test where a very high percentage of people are truly allergic.

That means you can't compare the numbers on RAST tests for different allergens—for one food, a number could represent a positive result, while for the other, the number means a negative result.

It's good to know that specific IgE values are usually reported in units of micrograms per milliliter (μg/mL). Some, however, are reported on a numerical rating scale (often, but not always, from 0 to 5 or 6). On rated tests, 0 almost always indicates an exceedingly low chance of a true food allergy, while the higher numbers indicate a very strong probability of allergy and a strong likelihood of a severe reaction.

It's possible for your doctor to use the results from your RAST to help determine some aspects of your treatment. RAST tests may help to determine whether a child is showing signs of outgrowing a food allergy. Although reliable predictive levels have been established for some foods, those levels sometimes vary with the patient's age, and researchers haven't determined predictive levels for all foods.

One study on RAST tests noted that many children had been told they couldn't eat foods they turned out to be able to tolerate during a food challenge. That's why RAST tests are generally given along with a careful medical history, skin testing where possible and, when appropriate, food challenges.

RAST tests are just one way of testing for food allergies. They may provide some valuable clues to help determine why you have symptoms, but your allergist is likely to use them in combination with other allergy tests. 

As always, if you have specific questions about interpreting your test results, or wonder why you were offered a particular medical test as part of a food allergy diagnostic workup, your allergist or immunologist is the best person to ask.