Topics in Allergy
Interpreting RAST (Blood Test) Results for Food Allergies
A RAST (Radioallergosorbent test) is a way of measuring the amount of IgE antibody in the blood stream made against a specific allergen. RAST tests and allergy skin tests give similar information. Both identify IgE antibody (allergy antibody) against materials you might be allergic to. RAST tests must be done by specific methods to be accurate and therefore must be sent to specific laboratories. RAST tests and skin tests each have their advantages and disadvantages. Skin tests may pick up some allergies that would be missed by RAST tests but skin tests have a small risk of precipitating an allergic reaction.
RAST tests report IgE made against specific foods in units known as KU/L. Higher levels indicate a higher chance you will have a reaction if you eat the food. Lower levels indicate a lower chance you will have a reaction.
Traditionally RAST TESTS have been reported as CLASS 0 to CLASS 5 or 6. Class 0 indicates no allergy. Class 5 or 6 indicates high allergy
CLASS 0 (less than 0.35 KU/L)
CLASS 1 (0.35-0.7 KU/L)
CLASS 2 (0.71-3.5 KU/L)
CLASS 3 (3.51-17.5 KU/L)
CLASS 4 (17.51-50 KU/L)
CLASS 5 (50.01-100 KU/L)
CLASS 6 (greater than 100 KU/L)
The significance of a result (the likelihood of reaction the next time you eat a food) varies with the degree of reaction on the test and with the food. For some foods the predictive value of the RAST test is very high, for others it is lower and not as clear cut as we would like it to be. At times, additional skin testing and in office challenges are required to clarify the significance of a test.
Recently the significance of different levels of IgE found on RAST tests for certain foods has become clearer and this has been useful in interpreting RAST test significance.
Examples of RAST test significance include:
Egg RAST greater than 7KU/L (greater than 2 KU/L in infants under 2 yrs) indicates a 98% chance one will react when eating eggs (2% do not react).
Milk RAST greater than 15 KU/L (greater than 5 KU/L in infants under 2 yrs) indicates a 95% chance one will react when ingesting milk (5% do not react).
Peanut RAST greater than 14 KU/L indicates a 100% chance of reacting when eating peanuts. (A RAST of 1 KU/L indicates a 50% chance).
Fish RAST greater than 20 KU/L) indicates a 100% chance one will react when eating fish.
Tree nut RAST greater than 15 KU/L) indicates a 95% chance one will react when eating tree nuts (5% do not react).
Soybean RAST greater than 30 KU/L) indicates a 73% chance one will react when eating soy-although some forms of soy may be tolerated while others may cause a reaction.
Wheat RAST greater than 26 KU/L) indicates a 74% chance one will react when eating wheat. Wheat is often positive at low levels in people who are allergic to grass pollen and may not cause reactions in that situation.
RAST TESTS AND SKIN TESTS TO LEGUMES can be difficult to interpret. Legumes include peanut, beans, pea, soy, lentil. These often cross react on RAST or skin tests but patients often have problems with only one of the legumes when ingested, despite the positive tests. This is most often seen in people allergic to peanuts that show positive tests to several legumes but have no trouble ingesting legumes other than peanuts. We sometimes need both RAST tests and skin tests followed by in office challenge with specific legumes to clarify this question. Lentils, beans, peas usually cause mild reactions of mouth itching, unlike peanuts and soy that more commonly cause more significant anaphylactic reactions.
RAST tests for many fruits & vegetables give many false negative results and skin testing with commercial extracts and with the fresh food are often required.
Patients can have anaphylaxis (allergic reactions) with RAST tests results less than 0.35 KU/L. If the history is suggestive of anaphylaxis additional skin testing and food challenges in controlled settings may be indicated.