How Can I Interpret the Numbers on my RAST Test Results?

Laboratory test result
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Question: How can I interpret the numbers on my RAST test results? What do they mean?

Answer: A RAST test (RAST stands for "radioallergosorbent test") measures the level of allergen-specific IgE antibodies in your blood. That is, the test measures the concentration of antibodies your body has created to combat a particular food allergen.

Few doctors are using the RAST blood test any longer 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.

RAST Test Results: Tricky to Interpret

The test measures specific antibodies against specific allergens. However, not everyone with the same concentration of allergen-specific IgE in their blood will react in the same way to the presence of an allergen in their bodies (in medical terms, this is known as in vivo — literally, in the flesh).

Someone with a relatively low concentration of one particular allergen-specific IgE may have a fairly severe reaction when actually encountering that allergen in the flesh, while someone with a higher concentration of that specific IgE may not react at all to the allergen.

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, though this varies by allergen.

In general, lower rates of allergen-specific IgE are correlated with a lower chance of reacting in a blind food challenge or in a real-world setting, especially if skin test results are negative and if there's been no history of reaction to that particular food.

Your Actual RAST Test Results

Different foods have different specific IgE levels that are considered "predictive," meaning they're predictive of you having 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.

This means that identical numbers on a RAST test for two different foods with different predictive IgE levels may produce different results: for one food, that number could represent a positive result, while for the other, the number means a negative result.

Specific IgE tests are usually returned in units of micrograms per milliliter (μg/mL).

Some, however, are returned 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 normally mean very strong probability of allergy and a strong likelihood of a severe reaction.

Your allergist may use these results to help determine some aspects of your treatment — to see whether a child is showing signs of outgrowing a food allergy, for instance.

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 noted that many children had been told they couldn't eat foods they turned out to be able to tolerate on 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. 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.

Sources:

Hamilton, Robert. "Chapter 70: Laboratory Tests for Allergic and Immunodeficiency Diseases: Principles and Interpretations" Middleton's Allergy: Principles and Practice. 6th Ed. Philadelphia: Mosby, 2003.

Kurowski, Kurt and Robert W. Boxer. "Food Allergies: Detection and Management." American Family Physician. June 2008, 77(12). 1687-88.

Sicherer, Scott. "Advances in Allergic Skin Disease, Anaphylaxis, and Hypersensitivity Reactions to Foods, Drugs, and Insects in 2007." Journal of Allergy and Clinical Immunology. June 2008, 121(6). 1351-58.

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