Blood tests vs. Skin tests

Posted on: Wed, 04/21/2004 - 12:23am
Anonymous's picture
Anonymous (not verified)

We just had our youngest daughter retested last week by blood test and honestly don't know what kind (Rast, Caprast or something different), but do know that it was IGE specific. (My eldest has been tested by IGG and IGG4, as well as IGE.) I don't think it was Rast because within the visit with the ped. allergist said he had a used Rast machine if we were interested (as a joke), so I am thinking it was some other kind of allergy test. I will try to find out when I call the nurse tomorrow. Anyway, she came up negative for everything. Now, back in November she was skin tested for things and came up strongly positive (based on the wheel size) to peanut, egg, soy, corn, rice, chicken and grass. Her symptoms include some hives here and there, lots of sneezing and rubbing her nose and some loose stools (this only happens occasionally). Now, she has not been expose to the peanut or egg, so the above symptoms are possibly for the other things. She has a much different symptom history than my oldest child and I don't think her allergies are as severe as her sister's allergies.
Sorry, this question got lost, but which test do you go by? Can both kinds of tests have false negatives and positives? Just confused once again, and when you think you are feeling comfortable, boom, something else just doesn't make sense.
I just got her a medic alert too for the peanuts and eggs just like my oldest, and strongly feel I should just keep it that way. I am looking forward to anyone's insight on this issue.
TIA [img]http://uumor.pair.com/nutalle2/peanutallergy/smile.gif[/img]

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Posted on: Wed, 04/21/2004 - 12:55am
NUTTYMOM's picture
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Joined: 04/20/2004 - 09:00

WE JUST RECENTLY WENT THRU SOME OF THE SAME PROCEDURES. OUR ALLERGIST TOLD US THAT AGE CAN PLAY A KEY FACTOR ON THE ACCURACY OF SKIN TESTS. OUR DAUGHTER IS 2.5 AND HAD SKIN TESTING AND HAD BLOOD RAST TESTS. THE DOCTOR SAID IT IS VERY LIKELY TO GET FALSE NEGATIVES ON SKIN TESTS, BUT THERE ARE NEVER FALSE POSITIVES USING THIS METHOD. THE WAY I UNDERSTAND IT IS IF THE SKIN TEST IS POSITIVE, THERE IS AN ALLERGY. IF IT IS NEGATIVE, FURTHER BLOOD TESTS MAY BE NEEDED TO BE SURE. OUR DAUGHTER TESTED NEGATIVE FOR EVERYTHING BUT PEANUTS ON THE SKIN TESTS, BUT WAS POSITIVE FOR EGG WHITES AND WHEY WITH THE BLOOD TESTS. HE RECOMMENDED WE RETEST AGAIN IN A YEAR SINCE SHE WILL PROBABLY OUTGROW ALL BUT THE PEANUT ALLERGY. HOPE THIS HELPS AND GOOD LUCK!

Posted on: Wed, 04/21/2004 - 1:07am
klrwar's picture
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Joined: 02/25/2004 - 09:00

This is what I've read/been told about blood vs. skin tests...
The blood test has a higher incidence of false negatives (where you really are allergic and it says you're not) compared to the skin test. It is also believed to be more accurate at diagnosing food allergies than environmental allergies. The Immunocap RAST is much better than the regular RAST -- it's much more accurate.
The skin test has a higher incidence of false positives (where you are really not allergic when it says you are) compared to the blood test. It is believed to more accurate at diagnosing environmental allergies than food allergies.
Although both tests assign "degrees of allergy" with their results (Class 0, 1, 2, etc. for the blood, 1-4 for the skin) they are only predictors of the LIKELINESS that a reaction will occur...not the severity of that reaction. For example, if you are a Class 6 RAST to peanuts, there's something like a 99.99% chance that you'll have a reaction if you eat peanuts...where if you are a Class 0 there's only a 12% chance or something like that (there's a chart posted on this board somewhere). However, what that reaction will BE (runny eyes vs. hives vs. throat swelling) is different based on the person.
Our doctor said the only true test of an allergy is a food challenge --- tests alone cannot say yes/no -- they only give you back up/more information to support a patient's reaction history. You can concievably have a person who is negative on both the skin and blood tests and still has allergic reactions to peanuts and is truly allergic. Likewise, you can also have someone that's off the charts on both tests that eats peanuts every day without a problem. Although both of these things are really rare statistically, they can/do happen -- according to my son's doctor.
Kristin

Posted on: Wed, 04/21/2004 - 1:17am
MommaBear's picture
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Joined: 09/23/2002 - 09:00

Quote:Originally posted by klrwar:
[b]This is what I've read/been told about blood vs. skin tests...
The blood test has a higher incidence of false negatives (where you really are allergic and it says you're not) compared to the skin test. It is also believed to be more accurate at diagnosing food allergies than environmental allergies. The Immunocap RAST is much better than the regular RAST -- it's much more accurate.
The skin test has a higher incidence of false positives (where you are really not allergic when it says you are) compared to the blood test. It is believed to more accurate at diagnosing environmental allergies than food allergies.
Although both tests assign "degrees of allergy" with their results (Class 0, 1, 2, etc. for the blood, 1-4 for the skin) they are only predictors of the LIKELINESS that a reaction will occur...not the severity of that reaction. For example, if you are a Class 6 RAST to peanuts, there's something like a 99.99% chance that you'll have a reaction if you eat peanuts...where if you are a Class 0 there's only a 12% chance or something like that (there's a chart posted on this board somewhere). However, what that reaction will BE (runny eyes vs. hives vs. throat swelling) is different based on the person.
Our doctor said the only true test of an allergy is a food challenge --- tests alone cannot say yes/no -- they only give you back up/more information to support a patient's reaction history. You can concievably have a person who is negative on both the skin and blood tests and still has allergic reactions to peanuts and is truly allergic. Likewise, you can also have someone that's off the charts on both tests that eats peanuts every day without a problem. Although both of these things are really rare statistically, they can/do happen -- according to my son's doctor.
[/b]
Thank you for sharing this.
I'm going to discuss this with my cubs' physician next visit. Currently have one older cub with a diagnosis of PA/Nuts and another younger cub with an [b]incomplete diagnosis of PA[/b].
Wonder if [i]he (youngest cub) is going to need[/i] a [b]diagnosis[/b] of PA in order to obtain an IHP or 504 plan for PA?

Posted on: Wed, 04/21/2004 - 10:03am
mommyofmatt's picture
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Joined: 03/12/2004 - 09:00

My allergist told me the same thing as Kristin's regarding skin vs. blood tests. It's nice when there's consistency isn't it?
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Meg, mom to:
Matt 2 yrs. PA,MA,EA
Sean 2 yrs. NKA

Posted on: Wed, 04/21/2004 - 1:24pm
Yonit's picture
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Joined: 06/24/2002 - 09:00

Same info as Kristin here in Chicago. A negative skin test is considered very reliable (at least for that point in time, as we know allergies can develop later). Our allergist feels that a positive skin test should also be accompanied by a "story" - that is, a real incident of a reaction, for it to be considered a true allergy. We do blood testing for peanut and latex, to get a sense of more specific levels (or changes) of antibodies and because the latex skin tests are highly unreliable and potentially dangerous. According to our doc, false negative are more likely for the blood tests.
[This message has been edited by Yonit (edited April 21, 2004).]

Posted on: Thu, 04/22/2004 - 12:03am
Anonymous's picture
Anonymous (not verified)

Thank you for all your replies. It is starting to make better sense now and we will see what our allergist tells us. [img]http://uumor.pair.com/nutalle2/peanutallergy/smile.gif[/img]

Posted on: Thu, 04/22/2004 - 3:56am
Anonymous's picture
Anonymous (not verified)

mom2nickie, glad some members were able to help you. [img]http://uumor.pair.com/nutalle2/peanutallergy/smile.gif[/img]
I have never had my PA son "officially" tested for PA, if that makes any sense. He did have skin prick testing done when his sister had it done (she is younger) and my son, in particular, came back with a LOT of false positives because he had ezcema on his back where the testing was done.
With my daughter, because the serum would have been considered her *first* exposure to peanut products, it was never really clear to me whether the test "counted" or not.
I do know that I found the whole thing terribly confusing.
Best wishes! [img]http://uumor.pair.com/nutalle2/peanutallergy/smile.gif[/img]
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