RAST VS. SKIN TEST?

Posted on: Sun, 09/03/2000 - 6:18am
amy2's picture
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Joined: 09/02/2000 - 09:00

My son had the RAST test when he was 10 months old after we noticed his mouth broke out in a rash after having a small amount of peanut butter. He tested positive for PA. I have learned from reading the boards, that nestles choc. chips have peanut traces. He never has had a problem with them. And his allergist suggested getting retested before he starts regular school to see if he still has the allergy. Should he get the RAST again, or the scratch test? His allergist says different specialists feel one way or the other about PA. Some say it can be outgrown, some say it can't. What is the latest thinking on this? And, my son is also allergic to about every environmental allergy you can think of. So, could his PA worsen? His symptoms have never been any worse than a rash around his mouth, but he has never been exposed to a large amount of Peanut. Is this too many questions? Sorry, I am new here, and am very interested in learning as much as I can! Thanks in advance!

Posted on: Mon, 09/04/2000 - 1:47pm
krasota's picture
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Joined: 04/24/2000 - 09:00

if your son hasn't been exposed all these years, his RAST might show a false negative. i have a false negative for my latex RAST levels, but a super positive skin/respiratory reaction.
just something to think about,
ygg

Posted on: Mon, 09/04/2000 - 11:52pm
anonymous's picture
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Joined: 05/28/2009 - 16:42

I agree with krasota. RAST tests can give a false negative. If your son does not have excema or other skin problems, I think I'd go with the skin prick test. Of course, I've read some horror stories on this board w/regards to anaphylactic reactions to skin pricks.
Yes, some people outgrow peanut allergies. There is usually a correlation between low Ige levels and outgrowing the allergy. Usually but not always. I know there is an extensive post on this website about outgrowing PA. Use the search button, or try to look for a post on the FAN conference.

Posted on: Mon, 07/16/2001 - 11:20pm
ImmunoCAPAllergyBloodTest's picture
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Joined: 07/16/2001 - 09:00

Dear Amy, et al,
my name is Lorraine and I am the U.S. Product Manager for Allergy & Instrumenation at Pharmacia Diagnostics, the maker of the ImmunoCAP Allergy blood test. Not all blood tests are the same. I have just posted information concerning this newer technology in some other sections of this site. Here is some information related to your very question.
Summary: A May 1999 study done by Dr. Wood and associates at Johns Hopkins University School of Medicine, revealed that ImmunoCAP testing exhibited excellent diagnostic efficiency when compared to skin prick testing. Intradermal skin testing added little to the diagnostic evaluation of allergy.
Annals of Allergy, Asthma, and Immunology; May 1999
Journal of Allergy and Clinical Immunology, Vol. 96, No. 5, Part 1, November 1995
***********************************************************
A Comparison of Skin Prick Tests, Intradermal Skin Tests, and RASTs in the Diagnosis of Cat Allergy
Robert A. Wood, MD; Wanda Phipatanakul, MD; Robert G. Hamilton, PhD; and Peyton A. Eggleston, MD
Journal of Allergy and Clinical Immunology, Vol. 103, No. 9, Part 1, May 1999
The Study
One-hundred twenty patients were challenged with a cat exposure model after first evaluating by history, skin prick tests (SPTs), intradermal skin tests (IDSTs) if SPT results were negative, and in vitro allergy testing using Pharmacia & Upjohn technology. Purpose of the study was to determine the predictive value of SPTs, IDSTs, and RASTs.
Conclusion
"Although both SPT and RAST values exhibited excellent efficiency in the diagnosis of cat allergy, IDST scores added little to the diagnostic evaluation." (pp. 773-779)
Editorial Highlights
SPTs and RASTs exhibited high levels of sensitivity, specificity, positive predictive values (PPVs), negative predictive values (NPVs), and overall efficiency. (p. 777)
The poor sensitivity, poor specificity, and positive predictive efficiency of IDSTs suggest they be used with caution and never be used alone as the basis for major therapeutic decisions.
For respiratory allergy, no gold standard for testing exists, although the authors suggest well-standardized and highly potent SPT as the most sensitive.
It is noted that potency of allergen extracts can result in variations in SPT results. Further, test methods used in the application of SPTs can vary, and it is imperative that clinicians select a method that has been adequately tested and standardized. (p. 779)
RAST displayed excellent sensitivity, specificity, PPVs, and NPVs.
RAST scores were indicated as somewhat less sensitive than SPT, although it was noted that sensitivity would have been improved had ImmunoCAP been used throughout the study.
While the authors acknowledge this study was based on cat allergen only, the expectation is that results would be similar across most, if not all, allergens. (p. 779)
You can order the full text of this article here [url="http://www.isitallergy.com/05_physicians/06_order.asp?num=99067.RPC1&id=25."]http://www.isitallergy.com/05_physicians/06_order.asp?num=99067.RPC1&id=25.[/url]
************************************************************
Studies on the Relationship Between the Level of Specific IgE Antibodies and the Clinical Expression of Allergy: Definition of Levels Distinguishing Patients With Symptomatic From Patients With Asymptomatic Allergy to Common Aeroallergens
Journal of Allergy and Clinical Immunology, Vol. 96, No. 5, Part 1, November 1995
The Study and Its Purpose
The American Academy of Allergy, Asthma, and Immunology identified a need for a quantitative assay to "facilitate better definition of the relationship between quantity of IgE antibody and the symptoms or risk of disease." This study was designed to determine whether, in patients with some evidence of allergy, results from in vivo tests and quantitative results from in vitro tests would allow clinicians to distinguish those patients with clinically significant allergic disease from those with no clinical symptoms.
Summary of Results/Conclusions
Sensitivity, specificity, and diagnostic values for the antibody assays were determined and compared with values obtained similarly for skin prick test (SPT) results.
Skin testing had a lower diagnostic value (sum of sensitivity and specificity) than the Pharmacia CAP System

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