False Skin test ? Rast showed Class 6

Posted on: Mon, 12/01/2008 - 4:47am
tlemkau's picture
Joined: 12/01/2008 - 10:51

We are so confused ,Tuesday before Thanksgiving , we took our 4 year old daughter to the doctor for her annual skin test , this time we were shocked the test on all items came back negative ,no reaction at all on her back this was a extreme shock for 4 years we have been told our daughter we extremely allergic skin test at 15mm , so you can imagine our shock, so once we regroup we watched two areas and nothing is happening ,15 minutes later still nothing ,so the doctor suggested a RAST test , well we got the result today and the result came back CLASS 6 (greater than 100 KU/L) , so what now ? the doctor is even confused and cant explain it , what he is offering is to retest the skin at no charge if it comes back again as a negative then re challenge the RAST , Has anyone been down this road please help

Posted on: Mon, 12/01/2008 - 7:36am
lj's picture
Joined: 01/26/2006 - 09:00

We have the exact opposite situation.
For years my son has tested positive on both the Rast and skin test, even though his rast was relatively low at 13, then 3, then 2 and now this year 0. I was shocked when they said 0. Well, they skin-tested him immediately and he had a very large positive.
Our dr was confused by these results. He has told me to err on the side of caution and consider him highly allergic.
I think in your situation I would have your child's rast rechecked and then another skin test. Then go from there.

Posted on: Tue, 12/02/2008 - 1:17pm
tunibell1974's picture
Joined: 10/30/2008 - 22:32

This is definitely a stumper...I think that most of the time when the SPT and the RAST conflict, it's a situation like [b]lj[/b] described (i.e. SPT+/RAST-)
I would absolutely insist they run everything again.

Posted on: Sun, 12/07/2008 - 9:53am
cathlina's picture
Joined: 06/29/2001 - 09:00

Just wondering if your daughter had any antihistamines for a week or two before the skin tests? This can cause skin testing problems.

Posted on: Sun, 12/07/2008 - 2:08pm
cristym's picture
Joined: 11/19/2007 - 17:26

There can be false negatives to SPTs. I had something similar happen with my enviromental allergens this year. I react strongly to trees, so when I had a SPT test the Dr. was surprised that I had such a hard time this year with my enviromental allergies since I barely reacted to them on the SPT even though I had a large reaction to the histimine (control SPT). Since they were not Food Allergies...he explained that it would be extremely dangerous with FA's and should never be done with them, he offered to inject a small amount of the serum just under the skin to see if that would produce a reaction. It produced a Giant Welt, and then we did a rast which was also possitive.
Having a regular SPT done again sounds like a good next step for your daughter.

Posted on: Mon, 12/08/2008 - 8:00am
tunibell1974's picture
Joined: 10/30/2008 - 22:32

Does your daughter have eczema, and a high level of IgE in general? If so, this can really affect the tests. My daughter's skin prick tests were almost a total 180 from her RAST tests. Our doctor just thinks there is non-specific IgE binding. (I.E. she is non-reactive/tolerant to plenty of things that the test says she is "allergic" to.)

Posted on: Sat, 01/03/2009 - 10:26pm
logan's picture
Joined: 07/12/2008 - 12:42

I was wondering what you came up with for information. My son had like a 3mm wheal which is considered very small, I think, and a high RAST - 100 as well. The doctor thought he may be outgrowing his allergy until his blood test came back, when he said "not yet". I am still hopeful as he is not quite 4 yet.

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