Interpreting test results when the negative control gives a positive result
Warning: this post is nothing but dry technical wonderings about allergy test results. Feel free to skip it unless that sort of thing interests you.
After having an anaphylactic reaction to coconut last week, I dug out my copy of my allergy test results. When we did the skin prick tests, I knew very little about allergies and allergy testing at that point.
Looking back at my results last night, I was surprised to see that the allergist wrote down that it would require a 6/12 result to be considered a positive. (6mm wheal or welt, 12mm red area). Many allergists consider anything 3/6 or larger to be a positive reaction, but the general rule of thumb is that any reaction at least 3mm larger than the negative control is a positive.
###############
My reaction to the saline negative control on the food test was 3/9. Normally that would indicate dermographism, from what I've been reading, and make the tests much more difficult to interpret.
However, I don't think it's dermographism (a condition where a person gets a red raised welt from any scratch in the surface of the skin) because I did have several results that were truly negative, with no wheal at all.
When we did the environmental testing I had tests that registered 0/3 to 0/10 on 13 different items, so I know the positive result on saline can't be dermographism. If I had dermographism then I should have had welts on those 13 tests also. But my reaction to the saline on that test was 2/5.
In the food allergy test a month later my saline control got a wheal of 3/9. My smallest wheal (to almond) that day was 2/7.
Does that mean I'm reacting slightly to something in the saline? Or could it be that in both cases the saline control was directly below a substance I had a strong positive reaction to (on the food test the histamine and saline were next to each other)?
It seems that it would make sense to take the smallest wheal as the control rather than the saline, if the saline is that reactive but there are negative reactions with smaller wheals present, doesn't it?
The allergist said the test was negative for everything on the food test. I'm wondering if the results are even able to be accurately interpreted considering that my reaction to the negative control was a welt that would generally be considered a positive reaction.
My results on the environmental tests ranged from 0/2 to 20/35, with 24 of the 40 tests registering with a wheal of 3 or more, and 13 registering a wheal of 0. There were none that registered a wheal of 1 and only 3 (including the saline control) that registered 2. 16 of them the allergist considered positive because they had a wheal of 5mm or more. The histamine control was 7/30.
On the food allergy test, the histamine control was 8/50 and the saline control was 3/9.
My other results were: avocado (strong clinical reaction of immediate food-poisoning-like symptoms when consumed) 4/10, barley 4/7, chocolate 3/5, coconut (to which I later developed anaphylaxis) 4/5, corn 4/5, egg white 5/7, cow's milk (longstanding clinical symptoms of allergy/intolerance) 4/7, almond 2/7, hazelnut 3/6, peanut 3/5, oat 4/5, rye 3/3, sesame seed 3/5, shrimp 5/7, soybean 5/6, tomato 4/5, wheat 4/4.
The allergist (the one I really didn't like) said that the food test results were all conclusive negatives.
What does concern me a bit is that I get mild symptoms (mostly nausea/stomachache) when I consume shrimp, eggs and soy. Taking the smallest wheal (2/7 to almond) as the control would make the results for each of those foods positive instead of negative. I'm wondering if continuing to consume them frequently could be a bad idea. I'm not eating any of them currently for my nursing baby's sake anyway.
I found out this week that I do have a fairly close relative who nearly died from a seafood allergy, so I'm glad we've decided to avoid seafood for Baby E until she's older.
I have a lot of chronic symptoms that could certainly be allergy-related, and if I'm constantly consuming foods I'm reactive to, that could explain a lot of my chronic illness I would think. My official "diagnosis" includes fibromyalgia, chronic fatigue syndrome and IBS, but my symptoms definitely get worse when I'm exposed to my allergens (especially dust mites and milk).
I'm really curious what another allergist would say. It seems to me that when the negative control gets a positive result, it makes the whole test unreliable and difficult to interpret.
Purple_Kangaroo, allergic to coconut, avocado, milk, blackberry leaves, latex, many environmentals (mold, dust mites, pollen, cats, etc.) and most antibiotics
. . . Mom to 3 girls, including Baby E who is allergic to corn, soy, oats, kidney beans, apples, banana, mushrooms, mold and ???
After having an anaphylactic reaction to coconut last week, I dug out my copy of my allergy test results. When we did the skin prick tests, I knew very little about allergies and allergy testing at that point.
Looking back at my results last night, I was surprised to see that the allergist wrote down that it would require a 6/12 result to be considered a positive. (6mm wheal or welt, 12mm red area). Many allergists consider anything 3/6 or larger to be a positive reaction, but the general rule of thumb is that any reaction at least 3mm larger than the negative control is a positive.
###############
My reaction to the saline negative control on the food test was 3/9. Normally that would indicate dermographism, from what I've been reading, and make the tests much more difficult to interpret.
However, I don't think it's dermographism (a condition where a person gets a red raised welt from any scratch in the surface of the skin) because I did have several results that were truly negative, with no wheal at all.
When we did the environmental testing I had tests that registered 0/3 to 0/10 on 13 different items, so I know the positive result on saline can't be dermographism. If I had dermographism then I should have had welts on those 13 tests also. But my reaction to the saline on that test was 2/5.
In the food allergy test a month later my saline control got a wheal of 3/9. My smallest wheal (to almond) that day was 2/7.
Does that mean I'm reacting slightly to something in the saline? Or could it be that in both cases the saline control was directly below a substance I had a strong positive reaction to (on the food test the histamine and saline were next to each other)?
It seems that it would make sense to take the smallest wheal as the control rather than the saline, if the saline is that reactive but there are negative reactions with smaller wheals present, doesn't it?
The allergist said the test was negative for everything on the food test. I'm wondering if the results are even able to be accurately interpreted considering that my reaction to the negative control was a welt that would generally be considered a positive reaction.
My results on the environmental tests ranged from 0/2 to 20/35, with 24 of the 40 tests registering with a wheal of 3 or more, and 13 registering a wheal of 0. There were none that registered a wheal of 1 and only 3 (including the saline control) that registered 2. 16 of them the allergist considered positive because they had a wheal of 5mm or more. The histamine control was 7/30.
On the food allergy test, the histamine control was 8/50 and the saline control was 3/9.
My other results were: avocado (strong clinical reaction of immediate food-poisoning-like symptoms when consumed) 4/10, barley 4/7, chocolate 3/5, coconut (to which I later developed anaphylaxis) 4/5, corn 4/5, egg white 5/7, cow's milk (longstanding clinical symptoms of allergy/intolerance) 4/7, almond 2/7, hazelnut 3/6, peanut 3/5, oat 4/5, rye 3/3, sesame seed 3/5, shrimp 5/7, soybean 5/6, tomato 4/5, wheat 4/4.
The allergist (the one I really didn't like) said that the food test results were all conclusive negatives.
What does concern me a bit is that I get mild symptoms (mostly nausea/stomachache) when I consume shrimp, eggs and soy. Taking the smallest wheal (2/7 to almond) as the control would make the results for each of those foods positive instead of negative. I'm wondering if continuing to consume them frequently could be a bad idea. I'm not eating any of them currently for my nursing baby's sake anyway.
I found out this week that I do have a fairly close relative who nearly died from a seafood allergy, so I'm glad we've decided to avoid seafood for Baby E until she's older.
I have a lot of chronic symptoms that could certainly be allergy-related, and if I'm constantly consuming foods I'm reactive to, that could explain a lot of my chronic illness I would think. My official "diagnosis" includes fibromyalgia, chronic fatigue syndrome and IBS, but my symptoms definitely get worse when I'm exposed to my allergens (especially dust mites and milk).
I'm really curious what another allergist would say. It seems to me that when the negative control gets a positive result, it makes the whole test unreliable and difficult to interpret.
Purple_Kangaroo, allergic to coconut, avocado, milk, blackberry leaves, latex, many environmentals (mold, dust mites, pollen, cats, etc.) and most antibiotics
. . . Mom to 3 girls, including Baby E who is allergic to corn, soy, oats, kidney beans, apples, banana, mushrooms, mold and ???
Copyright 2009. All rights reserved.
Posted by purple_kangaroo at 10:45 AM[Permanent link]
6 Comments:
It sounds to me like you need a new allergist.
Big hugs to you and I hope that you can find some real answers for you and Baby E.
I had one allergist tell me I wasn't allergic to anything (any red areas were small and cleared quick). A 2nd allergist said I was allergic to a few things. I asked him to justify the different diagnosis. His response was he felt others were setting the bar too high and a reaction indicates an allergic response. He had studies from somewhere(I forget) to support his clinical findings. When he treated the people as if they had the allergy, even if it was a tiny response, their health improved. He treats CFS people. My CFS doc says we are more sensitive to things than normal so maybe the "general rule" isn't applicable for CFS patients. So, in your case, I'd say you know you got a reaction so adjust your life accordingly to avoid the items and you'll know soon enough if you start to feel better or not. After a long stable period you can always try to add things back one at a time in a controlled fashion to see how you react.
I had one allergist tell me I wasn't allergic to anything (any red areas were small and cleared quick). A 2nd allergist said I was allergic to a few things. I asked him to justify the different diagnosis. His response was he felt others were setting the bar too high and a reaction indicates an allergic response. He had studies from somewhere(I forget) to support his clinical findings. When he treated the people as if they had the allergy, even if it was a tiny response, their health improved. He treats CFS people. My CFS doc says we are more sensitive to things than normal so maybe the "general rule" isn't applicable for CFS patients. So, in your case, I'd say you know you got a reaction so adjust your life accordingly to avoid the items and you'll know soon enough if you start to feel better or not. After a long stable period you can always try to add things back one at a time in a controlled fashion to see how you react.
There is a huge amount of controversy about the validity of allergy testing. Some allergists claim that if you have been avoiding a food substance then you may not necessarily show a reaction on first challenge - so the wheal is not indicative of your response to eating it.
Plus, if you've been taking anti-histamines or quercetin-rich foods then this can interfere with the results.
In the UK, we don't do much allergy-testing and it is very hard to obtain immunotherapy. We have fewer than 30 allergy consultants in our NHS (our national medical system).
It's a very difficult area.
Regards - Shinga
One more data point since this stuff seems to be all over the map -- the ped allergist who tested Snuggly Girl told me the size of the welt had no correlation to the extent of the allergy -- essentially he felt the testing is binary. Reaction = allergy, no matter how large or small the reaction.
The reliability and opinions about allergy testing really seem to vary. Thanks for your comments, everyone.
Post a Comment
<< Purple Puzzle Place Home