Positive Skin Test but Negative Blood Test? Anyone else? — The Bump
Food Allergy

Positive Skin Test but Negative Blood Test? Anyone else?

We recently found out my son has an allergy to cashews and pistachios.  Long story short, my son has an egg allergy and bc of this we were told at our appt last year to hold off on tree nuts (bc of his increased risk of being allergic).  So fast forward to last week, my son is now almost 4 and we went in for a follow up. They did a skin test for all tree nuts and he showed a reaction to both cashews and pistachios.  We also did a blood test which came back showing his levels to be in the normal range.  I am just very confused.  I spoke to a nurse at the practice who told me that skin test results trump blood tests.  I am still waiting to talk to the doctor.  

I am just very frustrated bc my son never even had one of those nuts to even indicate being allergic.  So to be told based on conflicting tests to avoid those nuts is just very upsetting.  I of course understand that the doctors err on the side of caution, but I just don't want to believe he is allergic for some reason.  Did anyone else experience something similar and what happened?  Also anyone else have a child allergic to one or two tree nuts but still eats and tolerates other tree nuts?  Would love to hear similar stories, thanks!
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Re: Positive Skin Test but Negative Blood Test? Anyone else?

  • I am surprised that the nurse said the skin test trumps the blood test. The blood test gives a level of how allergic they are and I was told that was more accurate. My DD2 had a skin reaction to eggs but the blood test showed a very low level. So the dr suggested a food challenge (done in the dr office under constant supervision) and she had no reaction so they said she could eat them. That was two years ago and she hasn't had any problems. 

    She is also allergic to those same tree nuts and both her skin test and blood test came back high. So we have to avoid those tree nuts as well as the rest. We were told that with tree nuts there is a lot of cross contamination in processing so if you are allergic to one you need to avoid all.

     When you talk to the dr you could ask about doing an in office food challenge if they feel it's safe. It's a pain and we were there for 4-5 hours but so worth it. They basically put us in a playroom with a nurse and gave her a thumbnail amount of what she's allergic to. Then wait and watch. Then give a little more if there is no reaction. Then watch. They work it up so at the end she was eating a whole bowl of eggs and had been there 4 hours with no rash or reaction. 

     Good luck I do agree that you want to be on the safe side but find it odd that the tests are showing different results.
  • It is my understanding that blood tests show the likelihood of a reaction. The skin tests actually show the reaction. Thus, they are more reliable.
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  • Cashews and pistachios have similar proteins, so that's probably why he had a reaction to both of them. Those are the two tree nuts that my DD is most allergic to, but we avoid all tree nuts because of cross-contamination. Like the PP said, if the blood test was negative you could see if the allergist would do a food challenge.
  • jrrsjrrs member
    edited October 2013
    Both tests have rates if error. I used to believe skin was best because when i was young and the blood test had just been developed it was run on me. And it was highly inaccurate. But then DS1 had a ana reaction to pecans -by touch- and tested negative to them on the skin test but his blood numbers were through the roof. So who knows. Thats why they get both tests to complete the full picture. And a history. I agree with the food challenge.
  • DS tested negative to peanuts on skin and blood tests, but he had a reaction (hives and vomiting) when he tried peanut butter and he also got a couple hives when he ate a cookie that had been processed on same equipment as peanuts.

    He had FPIES to soy and his twin sister has FPIES to milk, that type of allergy doesn't show up on tests. But, the type of reaction he had to peanuts is classic and typically shows up on tests. But, of course, a reaction trumps all so we are peanut free until a food challenge in about 6 months.

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