Need Opinions Please on Test Results


One of my best friend's DD is 6 years old and has seasonal allergies--she takes Zyrtec but not allergy shots. When the allergist first tested her for seasonal allergies one year ago, the allergist also tested her for the big eight food allergens because DD had a mild reaction after eating a shrimp appetizer at a Japanese restaurant and all (skin and RAST) came back NEGATIVE. My friend took her daughter today for a follow up visit and brought the lemon extract that the restaurant used on the shrimp. The allergist went ahead and skin tested her daughter for several foods again, and this time peanut, shellfish, soy, and a few others came back POSITIVE. All in all 8/14 tested came back positive. The lemon extract was negative. The allergist ordered a RAST and she will know the results in 7-10 days.

The kicker is that my friend's DD eats peanuts, peanut butter, shellfish, etc all the time. In fact, she ate a whole bag of roasted peanuts on Sunday and a peanut butter granola bar today (the same day she tested positive for PA). The allergist told my friend not to panic yet and said it could be a false positive, or that she is just now developing the allergy.

Do any of you have experience with this type of scenario? False positives? Development of PA in an older child? Does the RAST test ever provide false positives? I am just seeking opinions and experience here--I know the test results and the allergist make the ultimate call, but I told my friend how wonderful and knowledgeable everyone is here and thought you might have some ideas!

On Jan 16, 2006

I don't know anything about false positives, but my DD will be 19 yrs next month, and just started to react to peanut butter in Nov. She is TNA (10 yrs) but took PBJ to school almost every day. Her scratch test 2 weeks ago was unmistakeable.

On Jan 16, 2006

The RAST test can give false positives if the overall IGE level is over 1000. This was clearly stated on my son's last RAST test (his overall IGE was 1690). However, the results also stated that any false positives would have been in any lower end scores or 1 or 2. Anything with a 3-6 you can be sure is a true allergy.

My son constantly developed allergies to things that he had previously eaten with no problems. This when he was quite young however. He ate wheat for 6 months before developing allergic reaction. Same with beans. Now he is extremely allergic to both.

On Jan 17, 2006

This is also what we have heard-- the more highly atopic you are, the more likely you are to have circulating antibodies to... everything. That doesn't translate into clinical symptoms NECESSARILY, but it could.

Scratch tests can also give false positives with certain people. (Like me-- I have serious dermatographism and a history of eczema, and so does DD.) We have been told that scratch tests in general are much better NEGATIVE predictors. So if you don't react, this is about 90% predictive of not having a clinical allergy. But even this isn't a sure thing, because if you react to a protein fragment or only to the intact (uncooked) protein these can be falsely negative. The same is true of the RAST, by the way. If your antibodies recognize a different 3-D portion of the protein than is included in the affinity probe (which is like a lock in search of keys to fit it) for the test, the test may not pick up IgE even if it is there.

Hope this helps.

On Jan 17, 2006

Our allergist told me that:

The skin prick test is more likely to give false positives.

The RAST is more likely to give false negatives.

So what we can conclude from that is that a positive RAST is probably a real allergy, and a negative skin prick test probably indicates the absence of an allergy.

In your friend's case, the tests are simply inconclusive. Your friend could try to remove the "positive" items from the child's diet temporarily to see if the "environmental" allergy symptoms improve. If they do improve, one or more of the foods could be the culprit. To figure out which one(s), add the foods back into the child's diet one at a time (one a week) to see if the allergy symptoms return. The child's reactions may be delayed or in the form of a runny nose, etc.


------------------ Mom to 6 1/2 yr old PA/TNA daughter and 3 yr old son who is allergic to eggs.

On Jan 17, 2006

I had a similiar question for my DD's allergist when her first RAST test came back positive for the foods she's allergic to...namely peanuts, eggs, bananas and rice. The dr. explained to me (and if you search here you will find threads that explain it far better than me!) that with a true allergy, and not an intolerance, a person will not necessarily react the first "X" amount of times they are exposed to said allergen. But, each time they are exposed, whether they experience a reaction or not, their tolerance to avoiding a reaction, or decreasing the severity of their allergy, is diminished. In addition, it may take 10 exposures of an allergen to cause a reaction in 1 individual. In another person, with a similiar level of allergy, it may take 100 exposures. Unfortunately, one does not know what that magic # of exposures will be so strict avoidance is key for one's safety. In addition, it's beleived that strict avoidance may also contribute to an overall decrease in that allergy's level. Hope that helps a bit.

On Jan 17, 2006

When my PA son was younger he tested positive to soy. Mind you, he was eating soy everything since he was allergic to dairy at the time. MD said to ignore it - consider it a false positive, but reduce the amount of soy in his diet. (BTW, his dairy test was negative, but the clinical symptoms were unmistakable, so the MD said to consider him allergic to dairy. He outgrew it at about age 5.)

Fast forward to a month or so ago (about 9 years later) when my son tasted some soy milk for the first time in ages. Result - big, fat swollen lips full of hives. Just wonderful. [img][/img]


On Jan 17, 2006

Our DS is PN/TN allergic, so I took our DD to get tested this past summer since she was starting preschool in the Fall. I had pretty much avoided giving her straight peanut butter because of her brother, but had eaten "may contains" and some items with peanut in them with no problems. She had a positive skin test so the allergist suggested we do a food challange. She ingested a tablespoon of peanut butter over a three hour period with absolutely no problems. The allergist said the skin test was false-positive.