Specific IgE Analysis
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.
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.
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
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)
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
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
of RAST test significance include:
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).
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).
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).
RAST greater than 20 KU/L) indicates a 100% chance one will react when eating
nut RAST greater than 15 KU/L) indicates a 95% chance one will react when
eating tree nuts (5% do not react).
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
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.
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
tests for many fruits & vegetables give many false negative results and
skin testing with commercial extracts and with the fresh food are often
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