The false negative rate to egg was 2 - 3%.  So the chance that she was sensitized to egg by the skin test was 97 - 98%.  So yes, there is a remote 2 - 3% chance that she was not really sensitized to egg by a skin test.  But chances are 97 - 98% that she was sensitized by a skin test.  Medical science is about objectively assessing a situation. If one explanation has a 98% chance of being correct and another explanation has a 2% chance of being correct, a good doctor will go with the explanation that has the 98% chance.  After all, medicine is a science.

DS was allergic to egg and then had a brief time when he could eat some forms of it.  Now he can tolerate very tiny amounts of baked egg.

The fact is that skin and RAST testing is not 100%.  The fact that your child tested neg and then reacted to me means it was a false negative.  (shrug)

My child tested neg to hemp but he was clearly already allergic to it.  Later he tested positive.  Neg test results are not 100% accurate. 

I would guess that many kids get sensitized in utero.  Many react upon first exposure.  My son reacted his first day of life while nursing. 

It depends on the allergen.  The false negative rate is anywhere from 2% to 15% depending on the allergen.  For example, a cap rast has a much higher false negative rate to tree nuts than any of the other top 8.   I don`t know about hemp.  There probably are not alot of studies on tests for hemp allergy.  As far as the egg, your ds passed an egg challenge.  Therefore, his first skin test which was negative was correct.  It can take more than one exposure to become allergic.  People can become allergic at any time, even adulthood.  It sounds like your ds was truly not allergic to egg at one point, since he passed a challenge.  After all, the challenge is the "gold standard".  He then became allergic to egg after tolerating it.  That can happen.  It doesn`t sound like his first skin test to egg was a false negative.   That would be different from dd`s situation when she was sensitized by the skin test.

My son is allergic to hemp, something that he initially tested negative even though he was already having reactions to it.  I started using hemp protein powder DS was in elementary school.  As soon as I started using it DS started having mystery reactions.  I made my breakfast with the hemp protein powder as I made DS's breakfast.  Some hemp powder was on my hands as I touched DS's toast and other breakfast foods and as I made DS's lunch.  Some got onto the table where DS ate.  DS had had over 20 mystery reactions that caused him to miss school or be late to school or be sent home from school in a matter of months.  I suspected hemp and started to be more careful with it but since I didn't know for sure I still got it in his food sometimes by accident.  The allergist tested him and he tested negative.  I fed him hemp seeds and he had anaphylaxis.  I stopped all hemp products and the mystery reactions stopped completely.

DS reacted to it but had a negative skin test even though he was clearly already exposed, sensitized and reacting to hemp.  He now tests positive to it.

DS also had a neg skin test to egg.  He challenged, passed and could eat it at home.  Then he had ana to it at home.  I am interested in seeing how he will test now.

Skin testing is only about 90% accurate for negative results.  It is not 100% accurate.  Almost 10% of people who are allergic test falsely negative.  I think that's important to know although, yes, it is a tangent topic to this thread. 

He skin tested her after the reaction and she was positive.  Her allergist said that if it were a false negative (at that point in time the false negative rate to egg was 2 - 3%), then the repeat skin test probably would have been negative also.  I also know people who have reacted during what was supposed to be their first exposure.  Usually that means that they had an exposure that they don`t know about---maybe an item made on shared equipment and not stated or otherwise cross contaminated.  We discussed this in detail.  The allergist said that if dd had already had an unknown exposure before the skin test, then it should have been positive.  There are many details to this story and how he concluded that she was sensitized by the skin test, but there is no point in hijacking this thread.  The point that is relevant to this thread is that dd was sensitized to egg by a skin test, which is a contact exposure.

That is true-- the first time DD had any egg-containing food (and it was traces in a mix), she had hives all over her body within a few minutes after just a couple of bites.  Less than a month after she anaphylaxed in spectacular fashion from her first known exposure to peanuts (~100-150 mg of pb). 

So I think that most of the time, it just isn't possible to identify with a great deal of precision exactly what a sensitizing exposure is.  Barring special circumstances, of course.  We're pretty sure that we know what sensitized DD to pistachios, for example, since she's very obviously never eaten them (even in trace amounts) in anything, given her PA diagnosis so young.  We think that one was from a shell that she found on our family room floor and popped into her mouth when she was about eight months old. 

Skin and blood testing are only about 90% accurate for negative results so it could have been a false negative.  Yes, I think the test could have sensitized her, too, but I know of many who reacted the first time they ate a food. 

Very interesting.  Dd was sensitized to egg by a skin test.  Dd was skin tested to egg and was negative.  Three days later I gave her egg for the first time and after two tiny bites she went into anaphylaxis. I had to use the epi.  I was really floored when it happened, since she had just skin tested negative.  The allergist said that the skin test probably sensitized her. 

Jeffrey Miller MD, FACAAI
Mission: Allergy, Inc.
Hawleyville, CT


http://www.acaai.org/allergist/Resources/letters/Pages/Anaphylaxis_from_Substances_Applied_to_the_Skin.aspx