Summary of allergy testing methods
Someone was asking about allergy testing for infants on a forum. They specifically asked how old a baby has to be to do muscle testing.
I replied with a summary of the various allergy testing methods. I'm posting it here in case anyone else might find it helpful. I will try to go through and add more links to studies and in-depth information later.
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Muscle testing, I don't know a whole lot about, though I know people who have had it done. I believe some practitioners will do it down to any age . . . I've heard of people having their babies tested by having the mom hold the baby and then the homepathic doctor tests the mother's muscle strength when in contact with a various allergen and that's supposed to tell you whether the baby is allergic to it or not.
Board-certified allergists/immunologists won't do tests such as muscle or electrodermal testing--only alternative practitioners do these tests and they are quite controversial. Some of the various controversial methods of testing are covered here. People swear this alternative testing works, but so far studies have not shown consistent or reproducible results even with the same person tested multiple times on the same type of equipment.
One method of testing that's been receiving a lot of attention lately is IgG blood testing, which is supposed to test for intolerances and delayed reactions. It's quite controversial also. Some studies seem to show a link between high IgG levels and allergic reactions, while others seem to show that IgG actually has a protective function--for example, with this study about environmental allergy shots they found that as IgE levels go down, IgG levels go up and reactions get less severe. Recent research suggests that IgG may be relevant in IBS and similar conditions. IgG testing is an area that's still under quite a lot of research and debate. Shinga at Breath Spa for Kids has been doing a series of posts looking at IgG testing and other mail-order home testing that's available--here's one, with links to others at the bottom.
There are a few kinds of testing allergists will do, that are commonly-accepted and well-supported in the scientific/medical community.
The main ones only test for IgE antibodies. A clinical reaction combined with positive IgE is your basic "true allergy" with immediate symptoms like hives, itching, swelling, severe GI reactions (many allergists only consider GI [gastrointestinal] symptoms to be allergy-related if they are seen in combination with other types of symptoms), wheezing, changes in blood pressure, etc. Allergies can range from mild to severe and the symptoms can vary quite a lot.
The most common method of detecting allergies is skin-prick testing, where they take an extract from an allergen and apply it to the skin with a tiny needle prick. Then they wait 15-20 minutes and measure the various spots to see if they developed raised welts or wheals. A wheal 3mm or more larger than the negative control is usually considered a positive (the negative control should usually not form a wheal).
With babies they usually do this on their backs (some allergists will test infants and others won't); with adults some allergists do it on the upper arm. You can test 15-30 or so different substances at a time this way, and you'll have the test results immediately. It's the cheapest, quickest and many consider it the most reliable method of allergy testing, so this is what most allergists will do.
Some allergists (one we saw included) do a different variation on skin-prick testing which involves scratching or causing abrasions in the skin. It is supposed to provide a larger wheal to allergic substances but also makes a wheal even to the negative control, which may obscure weak positive results or make results more difficult to interpret. That "push and twist" method wasn't as accurate for Baby E. Her allergies showed up accurately on tests with two different doctors with the first method, but not with this method.
There's also something called intradermal testing which involves actually injecting a small amount of the allergen extract under the skin, but it's not done much any more for food allergy testing--with a higher chance of both false positives and severe reactions, many allergists don't consider it worth it. Some still use it for testing environmental allergies.
Finally, there's a test called an IgE blood test (often called RAST) in which they draw blood and from that one blood draw they can test for many (over 100 sometimes) different allergens. They do this by mixing the blood with different allergen extracts and then seeing there are IgE antibodies that bind to the different allergens. This test is very expensive and takes a week or three to get results. Some resources say that it's less sensitive than skin-prick testing, but ImmunoLabs' ImmunoCAP test is supposedly comparable to SPT in accuracy.
Most of Baby E's allergies show up on regular skin-prick testing but not on IgE blood tests or the push-and-twist tests.
A few things to know: Some foods (especially fruits) are not as accurate when tested the typical way because the allergen extracts aren't as stable. Some allergists and researchers recommend doing a prick-to-prick test with certain foods (pricking the food directly and then the skin) rather than using commercial extracts.
Also, the size of the wheal doesn't necessarily relate to the severity of the reaction. You can have a small wheal and a severe reaction or vice versa.
Finally, IgE allergy testing won't identify delayed-reaction allergies such as the majority of eczema cases. Some practitioners will do patch testing (putting the food on the skin for a day or two) to test for delayed-reaction allergies, while others don't consider patch testing to be valid or delayed-reactions like eczema to be true allergies.
These tests also will not identify intolerances. Intolerances can have different symptoms from allergies, or can even have the same symptoms. There are things that look and act just like allergies but don't test positive on IgE allergy tests (either because it's a false negative or because it's not IgE regulated).
No method of allergy testing is 100% accurate. When I wrote this post, I didn't fully understand the difference between an allergy and an intolerance, so I need to go back and rework the post to reflect the fact that SPT and RAST are fairly accurate in diagnosing IgE-regulated reactions ["true allergies"], while not being reliable at all in diagnosing other types of reactions. With that caveat, there is some useful information about the reliablility and methods of different types of testing in the post.
The closest to fool-proof method of diagnosing an adverse food reaction, whether an allergy or intolerance, is by verifying through exposure to a particular food whether or not it causes a reaction. Depending on the severity of the suspected reaction, this may mean doing an elimination diet at home or an in-office food challenge. The "gold standard" for diagnosing adverse food reactions is a double-blind placebo-controlled food challenge. The post linked in the previous paragraph has more in-depth information about food challenges.
Both allergies and intolerances can have various causes and can vary greatly in symptoms and severity. Some intolerances are mild and dose-related, like lactose intolerance (not an allergy, but the lack on an enzyme that breaks down milk sugar). Some can even look and act like anaphylactic reactions but not be considered allergies by some allergists, because they don't test positive on IgE allergy testing.
That doesn't mean they're not real reactions to foods, but it does mean you'll have a harder time getting help from an allergist or getting some doctors to take you seriously if you have that type of intolerance. If you're interested in the difference between allergies and intolerances, I have a post with a lot more detail here.
We've run into that with Baby E . . . she has consistent, reproducible, severe reactions to certain foods (including rashes, swelling, itching and GI symptoms), but because she tested negative on IgE blood tests and doesn't get typical hives or asthma, we've had two allergists tell us that her adverse food reactions are not allergies and so they aren't interested/able/equipped to treat her.
The treatment is the same (avoidance of the food), the results are the same; it's only the terminology that's different. For Baby E's reactions, an oral antihistamine works to minimize many of them and we carry an epi-pen in case of a really bad reaction.
Anyway, that's a brief summary of some of the most common allergy testing methods. I hope some of the information is helpful to someone.
I replied with a summary of the various allergy testing methods. I'm posting it here in case anyone else might find it helpful. I will try to go through and add more links to studies and in-depth information later.
#######################
Muscle testing, I don't know a whole lot about, though I know people who have had it done. I believe some practitioners will do it down to any age . . . I've heard of people having their babies tested by having the mom hold the baby and then the homepathic doctor tests the mother's muscle strength when in contact with a various allergen and that's supposed to tell you whether the baby is allergic to it or not.
Board-certified allergists/immunologists won't do tests such as muscle or electrodermal testing--only alternative practitioners do these tests and they are quite controversial. Some of the various controversial methods of testing are covered here. People swear this alternative testing works, but so far studies have not shown consistent or reproducible results even with the same person tested multiple times on the same type of equipment.
One method of testing that's been receiving a lot of attention lately is IgG blood testing, which is supposed to test for intolerances and delayed reactions. It's quite controversial also. Some studies seem to show a link between high IgG levels and allergic reactions, while others seem to show that IgG actually has a protective function--for example, with this study about environmental allergy shots they found that as IgE levels go down, IgG levels go up and reactions get less severe. Recent research suggests that IgG may be relevant in IBS and similar conditions. IgG testing is an area that's still under quite a lot of research and debate. Shinga at Breath Spa for Kids has been doing a series of posts looking at IgG testing and other mail-order home testing that's available--here's one, with links to others at the bottom.
There are a few kinds of testing allergists will do, that are commonly-accepted and well-supported in the scientific/medical community.
The main ones only test for IgE antibodies. A clinical reaction combined with positive IgE is your basic "true allergy" with immediate symptoms like hives, itching, swelling, severe GI reactions (many allergists only consider GI [gastrointestinal] symptoms to be allergy-related if they are seen in combination with other types of symptoms), wheezing, changes in blood pressure, etc. Allergies can range from mild to severe and the symptoms can vary quite a lot.
The most common method of detecting allergies is skin-prick testing, where they take an extract from an allergen and apply it to the skin with a tiny needle prick. Then they wait 15-20 minutes and measure the various spots to see if they developed raised welts or wheals. A wheal 3mm or more larger than the negative control is usually considered a positive (the negative control should usually not form a wheal).
With babies they usually do this on their backs (some allergists will test infants and others won't); with adults some allergists do it on the upper arm. You can test 15-30 or so different substances at a time this way, and you'll have the test results immediately. It's the cheapest, quickest and many consider it the most reliable method of allergy testing, so this is what most allergists will do.
Some allergists (one we saw included) do a different variation on skin-prick testing which involves scratching or causing abrasions in the skin. It is supposed to provide a larger wheal to allergic substances but also makes a wheal even to the negative control, which may obscure weak positive results or make results more difficult to interpret. That "push and twist" method wasn't as accurate for Baby E. Her allergies showed up accurately on tests with two different doctors with the first method, but not with this method.
There's also something called intradermal testing which involves actually injecting a small amount of the allergen extract under the skin, but it's not done much any more for food allergy testing--with a higher chance of both false positives and severe reactions, many allergists don't consider it worth it. Some still use it for testing environmental allergies.
Finally, there's a test called an IgE blood test (often called RAST) in which they draw blood and from that one blood draw they can test for many (over 100 sometimes) different allergens. They do this by mixing the blood with different allergen extracts and then seeing there are IgE antibodies that bind to the different allergens. This test is very expensive and takes a week or three to get results. Some resources say that it's less sensitive than skin-prick testing, but ImmunoLabs' ImmunoCAP test is supposedly comparable to SPT in accuracy.
Most of Baby E's allergies show up on regular skin-prick testing but not on IgE blood tests or the push-and-twist tests.
A few things to know: Some foods (especially fruits) are not as accurate when tested the typical way because the allergen extracts aren't as stable. Some allergists and researchers recommend doing a prick-to-prick test with certain foods (pricking the food directly and then the skin) rather than using commercial extracts.
Also, the size of the wheal doesn't necessarily relate to the severity of the reaction. You can have a small wheal and a severe reaction or vice versa.
Finally, IgE allergy testing won't identify delayed-reaction allergies such as the majority of eczema cases. Some practitioners will do patch testing (putting the food on the skin for a day or two) to test for delayed-reaction allergies, while others don't consider patch testing to be valid or delayed-reactions like eczema to be true allergies.
These tests also will not identify intolerances. Intolerances can have different symptoms from allergies, or can even have the same symptoms. There are things that look and act just like allergies but don't test positive on IgE allergy tests (either because it's a false negative or because it's not IgE regulated).
No method of allergy testing is 100% accurate. When I wrote this post, I didn't fully understand the difference between an allergy and an intolerance, so I need to go back and rework the post to reflect the fact that SPT and RAST are fairly accurate in diagnosing IgE-regulated reactions ["true allergies"], while not being reliable at all in diagnosing other types of reactions. With that caveat, there is some useful information about the reliablility and methods of different types of testing in the post.
The closest to fool-proof method of diagnosing an adverse food reaction, whether an allergy or intolerance, is by verifying through exposure to a particular food whether or not it causes a reaction. Depending on the severity of the suspected reaction, this may mean doing an elimination diet at home or an in-office food challenge. The "gold standard" for diagnosing adverse food reactions is a double-blind placebo-controlled food challenge. The post linked in the previous paragraph has more in-depth information about food challenges.
Both allergies and intolerances can have various causes and can vary greatly in symptoms and severity. Some intolerances are mild and dose-related, like lactose intolerance (not an allergy, but the lack on an enzyme that breaks down milk sugar). Some can even look and act like anaphylactic reactions but not be considered allergies by some allergists, because they don't test positive on IgE allergy testing.
That doesn't mean they're not real reactions to foods, but it does mean you'll have a harder time getting help from an allergist or getting some doctors to take you seriously if you have that type of intolerance. If you're interested in the difference between allergies and intolerances, I have a post with a lot more detail here.
We've run into that with Baby E . . . she has consistent, reproducible, severe reactions to certain foods (including rashes, swelling, itching and GI symptoms), but because she tested negative on IgE blood tests and doesn't get typical hives or asthma, we've had two allergists tell us that her adverse food reactions are not allergies and so they aren't interested/able/equipped to treat her.
The treatment is the same (avoidance of the food), the results are the same; it's only the terminology that's different. For Baby E's reactions, an oral antihistamine works to minimize many of them and we carry an epi-pen in case of a really bad reaction.
Anyway, that's a brief summary of some of the most common allergy testing methods. I hope some of the information is helpful to someone.
Labels: allergies and adverse reactions
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Posted by purple_kangaroo at 10:20 PM[Permanent link]
4 Comments:
Jonathan Brostoff has a very readable work on this subject called _Food Allergies and Food Intolerances_. It is a definite "must read" for those with food issues.
Thanks . . . I'll check it out!
It is a fascinating subject, PK. For something which causes such inconvenience (and worse) to people there is surprisingly little available to help manage either allergy or intolerance (in the UK, anyway).
As you can tell, I'm less than enamoured of some of the testing that people have done. I accep that these direct to consumer tests will remain available, I just wish that there was more accurate reporting of their 'known' value.
Great post. Thanks. We are off to allergist for more testing tomorrow, or at least the discussion of more testing, this is a helpful refresher. And I had never heard of the scratch version before.
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