Allergy Testing

The diagnosis of pediatric allergy depends upon a number of key ingredients. It is essential that the patient have signs and symptoms of allergic disorders at the time and place of exposure. There must also be the demonstration of an antibody response to the suspected allergen.

The allergic disorders are anaphylactic shock, asthma, rhinitis (with potential complications of sinusitis), conjunctivitis, urticaria, angioedema, atopic dermatitis, and some cases of vomiting and diarrhea. There a number of medical problems that have been thought to be due to "Allergy", but have never been proven. Some examples of these conditions include abnormal behavior problems and migraine headache. Pediatric allergy testing can potentially find a cause for only those conditions that have been associated with allergy. The tests look for sensitization to allergens. This sensitization leads to the formation of an antibody called IgE. This antibody latches on to mast cells and circulating basophils. The mast cells are found in great abundance in organ systems that manifest allergic reactions- around blood vessels, airways, the gastrointestinal tract, and the skin. Pediatric allergy tests look for the presence of this IgE antibody to specific allergens. There are a variety of ways to test for allergy.

A key point to remember about pediatric allergy testing is that it is used to confirm a suspicion of allergy. The Pediatric Allergist listens to the history and performs a physical examination. With this information, the allergy tests are then selected to confirm the diagnosis. The value of the allergy test is only as good as the history that supports it. To look at this from another perspective, a positive allergy test by itself does not make the diagnosis.

Key points to know about pediatric allergy tests:

• A pediatric 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 confirms or excludes specific sensitivity.

Types of pediatric allergy tests:

• Allergen challenge
• Skin test
• Epicutaneous
• Intradermal
• Blood tests

The most conclusive test is the challenge. This would be where the patient is exposed to the suspected allergen to see if symptoms of the illness occur. When the problem is due to allergy, the challenge is reproducible and usually it does not take a large amount of exposure to elicit the symptoms. The challenge will reproduce the reaction. The response may be a serious life-threatening reaction. Challenges are dangerous and are only performed under special controlled conditions. They are not done when the problem is anaphylaxis. In our clinic, challenges are performed to foods, drugs, vaccines, and local anesthetics. The presenting problem is usually a skin reaction with exposure to the agent.

Due to the dangers of the challenge, skin tests help confirm the diagnosis of allergy. Two types of skin tests are performed in our clinic. The most frequently used test is the epicutaneous or prick skin test. This involves the introduction of the suspected allergens into the top layer of skin using a plastic probe. These are usually performed on the back and do feel "pinchy". They are quickly applied. We then wait 15 minutes then "read" the results. For all tests, there must be place two control tests- one a positive control, the other a negative control. The positive control should result in a red mark with a lump or wheal in the center. With this type of reaction, the allergist knows that there are no medicines being used that will interfere with the test results. The positive control reaction also provides a standard of comparison to read the other tests. The negative control makes sure that the patient has non-specific skin reactivity. If the negative control looks like the positive control, then there is non-specific reactivity. All other results are potentially invalid.

A second type of skin test is the "intradermal" skin test. This is where the allergens are injected into the skin with a very small needle. These are very sensitive tests. They tend to have many false positive responses, they are expensive, they may result in systemic reactions, and they are painful. In our practice they are rarely used, but will be offered to help tease out highly suspicious reactivity. These should never be used for food allergy testing- they frequently yield falsely positive results.

There are also blood tests available. These are called RAST, VAST, MAST, FAST, or ELISA depending on the laboratory system used. When compared to skin tests, the blood tests for allergy are not as good. They can be close, but in no area of testing does the blood test surpass the sensitivity, specificity, and predictive value of the skin test. The results are frequently dependent upon the laboratory that is performing the test. The physician who orders allergy blood tests must have full confidence in the facility doing the test.

What do the test results mean?

Allergy tests are scored in a variety of ways. The blood tests are in classes 0 to 6 and skin tests are scored 0 to 4 plus.

The score really has no relation as to how allergic the patient is. How allergic you are is more a function of what happens to you with exposure. Who would you say is more allergic, the patient who went into anaphylactic shock with milk ingestion and had a 2 plus skin test to milk or the patient who had contact hives or urticaria from milk who had a 4 plus skin test to milk? One almost died, the other gets an itchy rash. I look at the test results as a an answer to the question- is there sensitization to this allergen? Yes or No?

The value of the test again is as good as the history that supports it. It also may depend on what tool is used to perform the test and the quality of the allergen extract used.

Also note that for foods, especially in the model of atopic dermatitis- a type of eczema, positive food tests are a 50-50 proposition. Half of the time the positive result may not mean anything clinically- the concept of a false positive. However, the negative food test has great "negative predictive value"- with a negative skin test there is very little chance that the food with cause a problem.

Who can be tested?

Parents are often told that the child is too young to be tested. This may have some truth, but deserves comment. The issue is more what the child is being tested for. The first manifestation of allergy is due to food. It shows itself as a form of eczema called atopic dermatitis. This illness may appear after the first three months of life. An infant with dry, itchy, flaky skin who is more than three months of age could be tested to food to help find an allergic trigger for this problem. So, yes the very young can be tested.

Now testing the very young child for pollen allergy is not warranted. This is a frequent concern of parents of children with nasal symptoms or asthma. The development of pollen allergy takes time. It evolves over two to three seasons. There may be other aeroallergens that would be more pertinent- the dander of dogs and especially cats, and the excreta of roaches and house dust mites. These are more perennial allergens and can lead to sensitization in the young child.

Now that the tests are positive, what happens next?

The most effective form of allergy treatment is avoidance. Allergy tests results can also be used to formulate allergen extract for immunotherapy or allergy shots.

How to prepare for allergy testing:

Do not use any anti-histamines prior to allergy skin testing. A good rule of thumb is to avoid them for the week prior to the test.