Allergy Tests

Introduction

This section covers:

  • The value of the allergy test
  • A Comparison of the types of allergy test that are available
  • What to do with the results from allergy testing

Hopefully this will help in dismissing the fears and trepidations children and families have about allergy testing.

The diagnosis of allergy depends on a number of key observations. It is essential that a child have the signs and symptoms of an allergic disorder at the time and place of exposure. There must also be teh demonstration of an antibody to the suspected allergen. The demonstration of the antibody is where allergy testing comes into play.

The conditions in which allergy testing may help include:

  • Anaphylactic shock
  • Asthma
  • Rhinitis (with potential complications of sinusitis)
  • Conjunctivitis
  • Angioedema
  • Atopic dermatitis
  • Some cases of vomiting and diarrhea

A number of medical problems have long been thought to be due to “allergy”, but the connection has never been proven. Some examples of these conditions include abnormal behavior problems and migraine headaches. Allergy testing can potentially find a cause of only those problems that have been associated with allergy (a type 1 immune hypersensitivity reaction).

Allergy tests look for sensitization to allergens. This sensitization leads to the formation of an antibody called IgE. This antibody latches on to mast cells. Mast cells are found in great abundance in organ systems that exhibit allergic reactions-around blood vessels, airways, the gastrointestinal tract, and the skin. Allergy tests look for the presence of this IgE antibody to specific allergens. There are a variety of ways to test for allergy. Some are good, some are not so good.

A key point to remember, and a theme that is frequently repeated, is that allergy testing is used to confirm a suspicion of allergy. A trained pediatric allergist listens to a child’s history and performs a physical examination. With this information the allergist selects allergy tests to confirm the diagnosis, however the allergy test result is only as good as the history that supports it. In other words, a positive allergy test by itself does not make the diagnosis.

Key points to remember about pediatric allergy tests,

  • An allergy test does not diagnose allergic disease-the test determines the presence or absence of specific IgE antibodies.
  • IgE is a necessary component of an allergic reaction.
  • The physician must decide if the signs and symptoms of the illness are consistent with allergy.
  • The allergy test is an adjunct-it confirmes or excludes specific sensitivity.

 

Why Allergy Tests?

Allergy tests are the most common concern we have in our allergy practice and the one with the greatest mystique. First and foremost, there must be a story to be sorted out. In the office of any allergist worth their salt,  a detailed history is essential. Once the history of symptoms and exposures is established, an allergist who knows the environment and with my prejudice knows children (my practice is predominantly pediatric) will be judicious about selecting those items that are relevant to the situation for testing. There is little or no value to an isolated test result. We are all taught in our allergy training programs that the value gained from any test for allergy is only as good as the history that supports it. The test, regardless of how it is performed, only tells us that IgE is being made or has been made at some time to that item. For a person to be allergic, there has to be a history and positive allergy tests that match that history. So allergy = story and relevant test results.

The term ‘atopy’ refers to the presence of IgE antibody- if a test for allergy is positive, the person is ‘atopic’ meaning they make the antibody. You can have IgE antibody and not have any symptoms with exposure. It is easy to see why history is so important in selecting allergy tests. This also reminds me to explain the concept of ‘allergy test panels’. These are extensively marketed for our primary care providers. I guess the thought is that, for one set price, it looks at a large number of sensitivities all at once. The problem is that frequently the panel contains items that the child is not exposed to or are not a part of the history. So more is done, possibly at more cost, that may provide unexpected results, that may lead to the need to see an allergist to sort out the situation. I tend to be a purist. I wish that allergy tests could be used to predict allergy. If that were the case, I would be writing this as I enjoyed perhaps my 10th year of retirement. Families have sent cats and dogs out of homes. They have indulged in expensive environmental control measures and they have been told to extensively alter diets leading to nutritional issues all due to test results that may have not been relevant.

When you have that history all sorted out and there is a consideration for doing some sort of allergy test, where do you go, who do you see, and which test is the best test? All very valid and important questions.

Who Can Perform an Allergy Test?

A board certified allergist/clinical immunologist can perform an allergy test. An allergist who is board certified by the American Board of Allergy and Immunology has been trained in pediatrics or internal medicine and has then gone on to pursue specialty training with further sanctioning of the credentials by having passed the board examination. In allergy the training is in both pediatric and adult allergy and the allergy boards qualify the specialist to see all age ranges. My own personal bias due to my training and affiliation with Riley Children’s Hospital is that a pediatric allergist will have expertise in many pertinent aspects and special aspects relative to children. It doesn’t hurt to ask about the primary specialty training background in making a selection for allergy care making sure it fits your needs and the needs of the child. There are others who offer allergy evaluations and do allergy testing. It is your health care dollar- be sure to ask.

Different Types of Allergy Tests

  • Allergen challenge
  • Skin tests
  • Blood test

Allergen Challenges

Then most conclusive test is the challenge. During an allergen challenge, the child is exposed to the suspected allergen to see if symptoms of the illness occur. When the problem is due to allergy, the challenge will reproduce the reaction and it usually does not take a large amount of exposure to elicit the symptoms. The response may be a life-threatening reaction, making challenges inherently dangerous. In our practice, we will only perform a challenge under special, controlled conditions. A challenge is not usually done when the problem is anaphylactic shock. We perform challenges to foods, drugs, vaccines, and local anesthetics. There are other choices for verifying the diagnosis of allergy.

A common concern is “What tests- blood tests or skin tests?”.  So what is the best test? Oddly enough it is the challenge. Expose the individual to the substance thought to be the cause or trigger and see what happens. In reality, the bottom line is what happens with exposure. The test tells us of the mechanism and may help with predicting a clinical course.Forgo the challenge, what is the next option? Here opinions will vary but the skin test is in my mind the gold standard for an initial evaluation. All the blood tests for allergy are compared to the results of the skin test.

Skin Tests

Skin tests in our practice is the introduction of a small amount of the allergen into the top layers of the skin. We use a skin prick test. The test are very cost effective and provide us with immediate results so a plan can be put together at the time of the visit. You will hear horror stories about skin testing. Back in the day, we used a metal nail-like object. I have been tested with this and with something appropriately called ‘scarification’ which was a sewing needle cut in half at the eye of the needle yielding two sharp planes that were stored in the allergen of interest and twisted on the skin during testing. This is no longer done. We have a plastic device, an individual probe the has a few small tines.

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This probe sits in the extract and is applied to first few layers of the skin. It is indeed a prick sensation and it does not draw any blood. There are a number of variations on this theme of skin test apparatus. The tests are placed on the back. After a 15 minute wait, the results are read and recorded. Here is another quirk from me- I firmly believe that if the situation is important enough to warrant a test, then I wil be reviewing that test myself. I read and measure every allergy test that I order and I interpret the results. This reminds me of another common question, “how allergic is he?” based on the size of the allergy test results. This is my take on this one- the test only tells us if IgE, the antibody that mediates allergy is produced and that answer is a yes or a no. The severity of allergy is a function of the history- for example a person who has hives when dog saliva touches the skin can have a massive positive skin test and a person who had shock to the sting of a yellow jacket may have a skin test about the size of a dime. Who is more allergic? Clearly the tests do not sort this out, it is what happens to the person in the field. An anaphylactic shock reaction is ‘more allergic’ than the person who has contact urticaria from the dog. The size does help with allergen immunotherapy- we would need to go slower with those who have very large skin test reactions.

The following is a video of the actual placement of the skin tests. In this clip, there were 50 items placed. Note how fast the procedure is. My thanks to the young man and his family for allowing us to film him during his evaluation.

Intra-dermal Test

Another form of skin test is the intra-dermal test. These tests are expensive, painful, and frequently falsely positive. In this form of testing a small injection of the allergen of interest is injected into the top few layers of skin. Many studies have shown that this form of testing does not advance the diagnosis of allergy and in our practice, they are rarely performed.

RAST: Radioallergosorbant Test

Now to one other form of allergy testing that really needs to be critically discussed: the ‘RAST’.

First is how to refer to this type of allergy test. I always chuckle when I hear of read about RAST tests. It makes me think of the sign that says ‘ ATM machine’. The ‘T’ in RAST refers to test- so it would be radioallergosorbant test test right? If I can teach the residents and students that rotate through the allergy clinic only one thing, it is the proper way to talk about this form of allergy testing.

RAST refers to the technology that was developed years ago. It is commonplace now to use the term for any allergy test that is performed in a laboratory. The analysis may no longer use radioactive tags in the  measurement.

RAST is nothing new, it has been around for many years. The technology and its use in research with proper extensions to clinical practice have evolved significantly. The test requires a blood draw and they tend to be expensive. They are marketed as panels where a large number of tests are run for a set price. The results only tell us that IgE is being made by the child towards this allergen. The presence of that IgE may have no clinical significance at all. This is why going over a history to decide what to test for is essential. It takes time to run these tests, so the results would not be available immediately. So the child has to be stuck for a blood for this, the tests are expensive, and there will be a delay in getting the results.

Next is trying to sort out what the results mean. This should always be done by a trained health care provider and always in the context of the allergy history. I have children sent with having alot or a little bit of allergy based on ‘RAST’ and I am never sure what that means. These types of tests are all compared to skin testing. Skin testing is the gold standard.

These tests have value and I do use them in my practice. Here are a few situations when I reach for a RAST:

1. When antihistamines have been taken and I cannot perform allergy skin testing.

2. When the child is combative and puts themselves, our nurses, or our attendants at risk for injury.

3. When I am following food allergy and I am looking for critical values to decide on doing a food challenge

Summary

1. The best test is the challenge

2. The history dictates what is tested for

3. Skin testing is the gold standard

4. Avoid panels

5. Appreciate the limitations of tests- their specificity, sensitivity, as well as the positive and negative predictive value