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How Can I Interpret the Numbers on my RAST Test Results? What Do They Mean?

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Updated May 13, 2014

Question: How Can I Interpret the Numbers on my RAST Test Results? What Do They Mean?
Answer:

A RAST test measures the level of allergen-specific IgE in your blood. That is, it measures the concentration of antibodies your body has created to against a particular food allergen. Interpreting a RAST result is slightly more complicated, however. This is because 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 vivo (that is, in the flesh).

Someone with a relatively low concentration of allergen-specific IgE may have a fairly severe reaction while someone with a higher concentration of allergen-specific IgE may not react at all -- that is, they're not truly allergic to a food even though the blood test comes out positive. (This is known as a false positive.) RAST tests have a high sensitivity and fairly low specificity, meaning that they have a low rate of false positives and a fair number of false negatives, though this varies by allergen. In general, lower rates of allergen-specific IgE are correlated with a lower chance of reacting in a double-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 a given food.

Different foods have different specific IgE levels that are considered "predictive." Where most people with a given level of specific IgE may react to one particular food, most people with that same level of specific IgE may not react to another food. Researchers determine these threshold levels by comparing RAST test results to the results of double-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 someone whose test results "look" identical for two different foods may be considered to have a positive result for one and a negative result for the other, if the foods have different threshold levels.

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 age, and researchers haven't determined predictive levels for all foods. A recent 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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