Question about RAST

Posted on: Sat, 07/28/2007 - 2:29am
cali96's picture
Joined: 07/13/2007 - 09:00

Reading through this board, it seems a lot of you or your kids have done the skin and RAST test. Can you tell me what makes taking the RAST necessary? On our initial visit with our allergist, we were informed that he would only give our son the RAST if he does not show any positive results with the skin test--which he did. We're supposed to go again in 3 months, though I'm not sure what for. I was wondering if we should ask for a RAST? Although from reading articles it seems neither the RAST nor the skin test can tell you precisely how allergic you are...unless of course I am reading wrong.

[This message has been edited by cali96 (edited July 28, 2007).]

Posted on: Sat, 07/28/2007 - 2:40am
tnmg's picture
Joined: 10/04/2006 - 09:00

Our allergist never mentioned a RAST test either but SPT can produce false positives. We found out about the peanut allergy 2 years after our son was diagnosed with milk and eggs. I asked for the RAST test just so we would have additional info on his allergies. Everything I've read has said neither test is 100% accurate, only food challenges are.
The other reason I asked for the RAST is to have a tangible number to track from year to year during his allergy testing. His milk and egg RAST numbers have dropped since last year, the peanut has gone up. SPT show 4+ (milk/egg about size of a nickle, welted lots of red, peanut bigger than a half dollar) on all three with peanut leaving a scar/scabby area for about 4 days.
Even though SPT showed 4+ for milk and egg, his RAST numbers for the two were low enough for the allergist to offer a food challenge. We'll wait on that challenge til he gets older.
Bottom line, you're right - no one test will tell you with all certainty. But its nice to see a trend.

Posted on: Sat, 07/28/2007 - 6:45am
chanda4's picture
Joined: 12/14/2006 - 09:00

I think they start with the scratch test(could be wrong) and if anything is positive, then they do the rast to basically follow the numbers. We've had testing every year, some years I don't do the scratch again(if no new symptoms or food worries) in that case I do ask for a caprast to just follow the numbers(climbing or falling etc...)
My kids have appt's next Tues(our yearly follow up) and my oldest I am not doing the scratch, but I am the caprast(to see her levels on the chocolate and beef)....but the other 3 I am doing both. I am testing some questionable foods on my 6yr old(like sun flower seeds, coconut, green beans(which he said his throat squeezed), and possibly some other legumes) 4yr old I am just going to recheck the peanut/treenut, he's been negative at previous testing(and check milk and egg levels)...and then my youngest, she has a some new ones deff going on.
I do know not all allergist will do both every year...the scratch test is more of a *yes* or *no* answer, then the caprast is more of a number-to-follow answer....neither predicts severity at all...I ask so they usually comply(so I can watch numbers)
Chanda(mother of 4)
Sidney-8 1/2(beef and chocolate, grasses, molds, weeds, guinea pig & asthma)
Jake-6 1/2(peanut, all tree nuts, eggs, trees, grasses, weeds, molds, cats, dogs, guinea pig & eczema & asthma)
Carson-4 (milk, soy, egg, beef and pork, cats, dog, guinea pig, grass, mold, dust mite and EE)
Savannah-1 1/2 (milk and egg)
[This message has been edited by chanda4 (edited July 28, 2007).]

Posted on: Sat, 07/28/2007 - 8:58am
SallyL's picture
Joined: 02/20/2006 - 09:00

From what I understand, our allergist does the RAST for the same reason - following the numbers (though he didn't do one the first year she was diagnosed). He initally did it because the welt was 'borderline' and did a RAST to check before he did a challenge. The RAST came back pretty high, so now he'll do RASTs without the SPT since the SPT wasn't very accurate for her.

Posted on: Sat, 07/28/2007 - 9:34am
alliedhealth's picture
Joined: 06/10/2006 - 09:00

I would encourage you to read the section of this large review article dealing with skin tests and RAST- it summarizes all the recent journal articles. Negative SPTs have good predictive value of lack of allergy (Unless recently taken longer acting antihistamines to my knowledge) where as positive ones have different predictive ranges depending on the size of the wheal, with greater than a 5-6 mm having good predictability of clinical reaction.
This article does reference if positive it is best taken in conjunction with clinical history which if significant also blood tests and challenge only if needed. The article references the studies better and more accurately than I can-
at the end of the section it outlines recent benchmark values for SPT/ IgE vaules have lowered the need for challenges due to higher predictability

Posted on: Sat, 07/28/2007 - 10:18am
jw's picture
Joined: 06/29/2003 - 09:00

DS's eczema gets way worse when we take him off antihistamines for skin testing, usually requiring oral steroids for a few days. All his initial tests for allergies were skin-based, but now we only do RAST. Even at age 5, he remembers skin testing enough to know he would much rather do RAST and is a trooper for them. He gets large reactions to skin testing, so as long as the RAST results stay high, I see no point in putting him through more of them. Our last RAST had 32 tests done (seasonal and environmental).
They ran 8 first, then based on those ran the rest. I don't think his back could have supported that. Also since that testing was to start immunotherapy, we needed tracable numbers. Skin testing for food can also still show positive for awhile after outgrowing an allergy. That could delay a challenge that could have been passed earlier if the RAST numbers were tracked and down to negative.

Posted on: Sun, 07/29/2007 - 2:29am
Sarahb's picture
Joined: 01/22/2007 - 09:00

From what I can tell following the numbers is just something to focus on but doesn't really mean what we may think it means. I have the feeling that these numbers fluctuate regularly and we don't know why or what that means.
We haven't had the RAST test and the allergist clearly didn't want to do it. Why would we do it? We know based on reaction and skin test that he is positive to peanut. I have had other foods tested based on suspicions and they were all negative. I can totally see myself wrapped up in the RAST numbers....but the only reason that I can see doing it would be look back when they figure out what they mean and try to learn something. I just don't think that they have as much value in diagnosis or what you do after diagnosis.
I'm sure in kids who have MFA and more complex conditions that they are more useful and I would probably insist on getting one.
Heck we had IgG testing done ....and the docs don't think that is valid. So what do I know?
I just don't know what a RAST test could tell me that would matter.

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