Negative blood test after positive skin test

Posted on: Sat, 04/19/2008 - 12:22pm
pri's picture
Joined: 04/19/2008 - 14:03

My son is 3 1/2 and has had 2 possible reactions to peanut butter. One when playing with peanut butter playdoh and he touched his eye and it swelled. The other eating a cheese peanut butter cracker and his lip swelled and blistered. He has had PB only a handful of other times, with no reaction noted.

We have been to the allergist and had the skin prick test. He was positive for peanuts (a 4+ on a scale of 0 to 4) and a 2 for tree nuts. We just got the blood tests results - I believe it was the RAST test - and it was negative for peanuts and tree nuts. The allergist said it was possible to have a positive skin prick and then a negative blood test. My understanding is that in order to have an allergic reaction you have to have the antibodies to the food, so how could his IgE number be 0 when he has had a reaction?

The allergist said to continue to avoid peanuts and she would repeat the blood test in 6 months. If that was negative then they would do a food challenge to be sure. Has anyone had an experience with a negative blood test after positive skin test??? Any advice would be much appreciated.

Posted on: Wed, 12/09/2009 - 10:22am
LinKay's picture
Joined: 08/17/2007 - 09:00

I really don't have any advice or thoughts on this. I do know this happened to a friend's child and her allergist said since he has a positive skin test and reacts to peanuts, he is still allergic and avoid all peanuts and may contains.
I have the opposite thing going on with my dd. She has a high rast for almonds. It is 11. We don't know what her skin test is because they always group the tree nuts together when testing. She ate toffee last week with no problems (it was an accident. I didn't realize toffee had almonds in it and fed it to her. I only checked for may contains or contains at the bottom-- big lesson learned). Anyway, she ate it and had always eaten almonds with no problem. but once she tested postive in the rast, I pulled it from her list of safe foods.
Now, the dr is saying to go ahead and let her eat almonds!
Scares me though. At what point will she have a reaction??? I feel like it is inevitable since she has such a high rast and allergies to other nuts.
Anyway, seems it is not black and white when it comes to allergies.

Posted on: Wed, 12/09/2009 - 12:38pm
diannek's picture
Joined: 08/05/2009 - 19:08

Thanks for your comments. We have since had a food challenge in the drs office. DS was given an extremely small amount of peanut butter (not even enough to cover the head of a straight pin). It took 12 minutes for the hives and itching to appear around his mouth. He was given a dose of xyzol (Rx strength benedryl) and another hour wait to be sure no other symptoms appeared. We left with a new epipen Rx and continue avoidance.
As for the tree nuts, will your dr test for individual tree nuts. Mine did. Thankfully, no tree nut allergies.

Posted on: Thu, 12/10/2009 - 3:16am
BestAllergySites's picture
Joined: 03/15/2009 - 21:46

Dianne and Linkay--I just posted 2 blogs on allergy testing here in the blog section.
Skin Prick and Blood tests have HIGH false positive rates. The only way to determine a true allergy is through history of reaction.
If a child has no reaction to food being eaten AND test results indicate an allergy--I would discuss a food challenge with your allergist.
If there is no history of reaction to a food--it should not be tested due to the high false positive rate.
Many people go to an allergist and don't know what food they reacted to. The allergist does a full allergen panel and each food shows up as positive. It if very rare to be truly allergic to 10+ items.
Eczema can cause false positive results on both skin prick (sensitive skin) and blood tests (causing high IGE levels).
Super high rast results (over 14-15) usually do indicate a potential allergy in the absence of a reaction.
However, under 14-15 and no reaction could mean a false positive.
Bottom line is--if your child can eat a food without reaction--he/she is probably not allergic.
As always--discuss with your allergist! :)

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