Re: Is the RAST test really better than skin testing or titrating for allergies

Jennifer,
	Thanks for writing to MadSci Network; I'll do my best to 
help you evaluate the different kinds of allergy testing.  First, a little 
background on the basic cause of allergic reactions.  Different people are 
allergic to different substances (called antigens) in varying degrees.  For 
instance, when I breathe in pollen, my immune cells overreact and produce 
large amounts of an unusual type of antibody known as IgE.  These IgE 
molecules bind to cells which release large amounts of chemicals known as 
histamines; it's these chemicals that cause typical allergy symptoms such 
as a stuffy nose and sneezing.  As you may know, some people have severe 
allergic reactions to bee stings or certain foods.  The histamine release 
in these people is so great that it swells the airway, and they need 
immediate treatment to be able to breathe.  
	Often people do not know what specific allergens are causing them to 
react.  As you noted in your question, there are several ways to test 
reactions.  As you will see, it is not necessarily that one method is 
better than the other.  Each method gives a patient different information 
about their allergies, and each has advantages and disadvantages.
	First,  when you visit an allergist, he will perform a skin prick test.  
This involves lancing the top layers of skin on your arm or back (You 
hardly feel this, and there isn't any blood shed!).  The doctor then 
applies a separate antigen solution to each lance mark.  After 10-15 
minutes, a mosquito bite-like itchy spot will develop in the places that 
contain allergens to which your body reacts.  The skin prick test gives 
your doctor yes or no answers and helps determine what your body reacts to; 
thus the skin prick test is a qualitative test.  
	A quantitative test measures the amount of antibodies in your blood 
that react to specific antigens.  RAST stands for radioallergosorbent test 
and was first developed thirty years ago in Sweden.  Breaking down the long 
name gives us clues as to what the test measures.  Common *allergens* are 
lined up in a grid pattern by *adsorbing* them onto a plastic plate.  
Patient serum (blood) is added to each section of the grid.  If the serum 
contains antibodies to the allergen, it will bind to the grid.  The grid is 
exposed to molecules that are *radioactive* and bind to human antibodies.  
Any sections of the grid that have bound human antibodies will show a 
positive signal.  You can envision a sandwich-like effect: allergen binds 
antibody binds radioactive molecule.
	RAST has undergone many changes since its development, but the basic 
detection principle remains the same.  In the early eighties, RAST lost 
support because it could not detect low levels of antibodies without 
sacrificing specificity.  However, the development of the modified RAST 
test (MRT) solved the problem, and forms of the test are in widespread use 
today. The test is also no longer radioactive, making it easier to use and 
cheaper.
	Doctors often rely on both MRT or RAST-like tests and skin prick tests 
to help determine the best course of treatment for allergy sufferers.  The 
skin prick test is quick.  While the RAST-like tests take longer, they 
offer more detailed results. 
	I hope this helps you evaluate the differences in allergy testing.  If 
you want to read a recent comparison of allergy tests, you can check out 
the following references:

Emanuel, IA. 1998. In vitro testing for allergy diagnosis.  Comparison of 
methods in common use. Otolaryngolgic Clinics of North America. 31(1)27-34.

Gordon, B. 1998.  Allergy skin tests for inhalants and foods. 
Otolaryngolgic Clinics of North America. 31(1) 35.

Good luck,
Tinsley