RAST vs Skin test

Posted on: Fri, 01/20/2006 - 7:47am
Wendi's picture
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Joined: 01/19/2006 - 09:00

If a person has a positive skin prink test but a negative RAST, is the person allergic to that item?

Thanks,
Wendi

Posted on: Fri, 01/20/2006 - 9:25am
gvmom's picture
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Joined: 08/24/2005 - 09:00

My younger son had a negative RAST for eggs (what he is allergic to) the last time he had one, but when given a skin test to verify, he was positive. The allergist still considers him allergic; we do too.

Posted on: Fri, 01/20/2006 - 9:31am
Corvallis Mom's picture
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Joined: 05/22/2001 - 09:00

Wow-- you said a mouthful there (or maybe typed a handful...)
What I mean is that this is probably the most ambiguous situation of all. (Even moreso than a positive RAST and negative skin test... which usually means "not allergic")
So what I mean by this is that skin tests are notorious for false positives... if a skin test is negative, this is (relying on bad middle aged memory here) something like 80%-90% good at predicting NOT ALLERGIC. A positive skin test is only about 30-50% predictive, on the other hand. Sometimes worse in certain people who have highly sensitive skin.
A RAST test only detects circulating antibodies. You can have an anaphylactic reaction that kills without enough IgE to detect on a RAST... but it would be rare. RAST is better at predicting how likely you are to be allergic... the higher the number, the more likely a clinical allergy is. On the other hand, we here know stories about very low RAST numbers (below 1.0) that correspond to immediate, severe reactivity as well as class 2 or even 3 being utterly non-reactive EVER.
If you have reaction history indicating immediate IgE mediated hypersensitivity (true allergy) then you're allergic under these conditions, JMO. Without any reaction history to go by, I'd assume allergic and check the RAST again in a year or two. KWIM?
Just remember, this isn't really medical advice, OK? What does your allergist make of your situation?
[img]http://uumor.pair.com/nutalle2/peanutallergy/smile.gif[/img] A warm welcome to you, Wendi. I hope that you find answers to your questions here.
[This message has been edited by Corvallis Mom (edited January 20, 2006).]

Posted on: Fri, 01/20/2006 - 1:01pm
jeancbpugh's picture
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Joined: 03/17/2001 - 09:00

we had neg skin test and neg RAST = anaphylactically allergic
------------------
Jean
DS allergic to peanuts & watermelon (9/2005 neg RAST peanut & watermelon; 9/2005 skin test neg peanut, positive watermelon; 12/2005 peanut food challenge=anaphylaxis)
other DS with pollen, animal, dust allergies

Posted on: Fri, 01/20/2006 - 10:57pm
TRexFamily's picture
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Joined: 11/30/2004 - 09:00

My DD had a Grade 4 respiratory reaction (with level 1-2 reactions in the other body systems) 12 days after the RAST came back negative for peanuts.
I copied this link to the chart so you can see how reactions are graded:
[url="http://pediatrics.aappublications.org/cgi/content/full/111/6/S2/1601/T2"]http://pediatrics.aappublications.org/cgi/content/full/111/6/S2/1601/T2[/url]
What does your allergist say about the results? What was the score on the skin test?
Anne

Posted on: Sat, 01/21/2006 - 2:11am
Wendi's picture
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Joined: 01/19/2006 - 09:00

Well I think our allergist is a bit nuts, so I am looking for a new one. She changes her opinion on things every time I talk to her... meanwhile I've got the whole family going there... not a good combo! One day she is telling me the skin test is more sensitive than the RAST and we should go by that over the RAST results, then the next day she tells me if the RAST is negative then the allergy has been outgrown.
My son, Luke, had a hive reaction the first time he tried eggs, but he also might have had other things going on inside. He is autistic and at that time was non-verbal so he could not tell me. He was ok with a few doses of benadryl. Then his skin test came back very allergic. That was 3 years ago. We could not get blood from him at that time.
At the same time, his triplet sister, Maddie, had the same reaction (also autistic and non-verbal at the time)... so basically from then on we have avoided eggs at our house. Her skin test came up even worse than my son's.
Since then we have done several skin tests. Maddie is severely allergic to eggs and now nuts and peanuts (which we had been avoiding anyway due to family anaphylactic reactions), confirmed by RAST.
But a couple of weeks ago, Luke came out positive for eggs on the skin. We finally got a RAST, but his RAST just came back negative. His peanut skin looked the same to me as the egg skin, but for some reason the allergist said it was negative and REFUSED to to a RAST. His control skin prick looked very different than the peanut prick, though both egg and peanut were a little less than the control histamine.
????
Meanwhile their triplet sister, Katie, is allergic to the sun (yes the sun) and something in Yoohoo drink (throat tightens) but she eats all the ingredients in other things and there is no reaction. Her skin testing all came out negative, even the histamine control (probably because she is on Zantac for GERD and it is chemically similar to an antihistamine) and her RASTs all came out negative.
Me... well my skin pricks came out positive for many foods, but negative to some too. The allergist is telling me that I have hypersensitive skin... but if that is the case, why did some come out negative right in the middle of all those positives? What I have patches of skin that are not hypersensitive? Most of my RASTs came back slightly positive, but not enough for her to diagnose an allergy to. My RAST for mold came back negative and then I had a mouth/throat reaction to a trace of my daughter's antibiotics on my finger when I obviously did not wash my hands well enough after spilling some on my hand, and licked my finger to turn a page a little while later. Before that I only had GI symptoms to antibiotics.
So I'm confused!!!!!!!!!
Thanks,
Wendi

Posted on: Sat, 01/21/2006 - 2:37am
Corvallis Mom's picture
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Joined: 05/22/2001 - 09:00

Wendi,
It really sounds like you need to get an allergist who can be a better advocate for your children. This is really critical with non-verbal children since they can't tell you what is going on at all. I know this was a much bigger problem when my child was too young to speak. Now, of course, she's not too shy about telling her allergist "that doesn't make any sense." when something doesn't.
I wouldn't know what to think given your experiences. I can see now why this is so confusing. A better allergist (someone with experience in FOOD allergies and better yet, pediatric food allergies) should be able to help you with this.
I am sorry that your current allergist isn't being very helpful to you.

Posted on: Sun, 01/22/2006 - 10:59pm
turtle's picture
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Joined: 12/10/2004 - 09:00

My son has not had any reactions, but had a positive skin test for peanuts when he was 2. A negative RAST test when he was 3. A repeat skin test last week at 3 3/4 and he was positive. My doctor says he is positive.
Confusing I agree.

Posted on: Sun, 01/22/2006 - 11:57pm
jtolpin's picture
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Joined: 05/28/2003 - 09:00

Heres a general rule.
If the child has a reaction to the food, he's allergic. Period. End of story. Thanks for playing.
A wise old woman once said to me, in my early yrs, and I preach it:
Reaction trumps Results.
Screw the tests if the child reacts.
Jason
------------------
[b]* Obsessed * [/b]

Posted on: Fri, 01/20/2006 - 9:25am
gvmom's picture
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Joined: 08/24/2005 - 09:00

My younger son had a negative RAST for eggs (what he is allergic to) the last time he had one, but when given a skin test to verify, he was positive. The allergist still considers him allergic; we do too.

Posted on: Fri, 01/20/2006 - 9:31am
Corvallis Mom's picture
Offline
Joined: 05/22/2001 - 09:00

Wow-- you said a mouthful there (or maybe typed a handful...)
What I mean is that this is probably the most ambiguous situation of all. (Even moreso than a positive RAST and negative skin test... which usually means "not allergic")
So what I mean by this is that skin tests are notorious for false positives... if a skin test is negative, this is (relying on bad middle aged memory here) something like 80%-90% good at predicting NOT ALLERGIC. A positive skin test is only about 30-50% predictive, on the other hand. Sometimes worse in certain people who have highly sensitive skin.
A RAST test only detects circulating antibodies. You can have an anaphylactic reaction that kills without enough IgE to detect on a RAST... but it would be rare. RAST is better at predicting how likely you are to be allergic... the higher the number, the more likely a clinical allergy is. On the other hand, we here know stories about very low RAST numbers (below 1.0) that correspond to immediate, severe reactivity as well as class 2 or even 3 being utterly non-reactive EVER.
If you have reaction history indicating immediate IgE mediated hypersensitivity (true allergy) then you're allergic under these conditions, JMO. Without any reaction history to go by, I'd assume allergic and check the RAST again in a year or two. KWIM?
Just remember, this isn't really medical advice, OK? What does your allergist make of your situation?
[img]http://uumor.pair.com/nutalle2/peanutallergy/smile.gif[/img] A warm welcome to you, Wendi. I hope that you find answers to your questions here.
[This message has been edited by Corvallis Mom (edited January 20, 2006).]

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