Diagnosing an allergy depends on a number of key indicators. The patient must have signs and symptoms of allergy at the time and place of exposure to the allergen. Patients must also demonstrate an antibody response to the suspected allergen.
Symptoms of allergic disorders may include anaphylactic shock, asthma, allergic rhinitis (with potential complications of sinusitis), conjunctivitis (eye inflammation), hives, angioedema (deep layer skin swelling), atopic dermatitis (skin irritation) and some cases of vomiting and diarrhea. Some believe migraines and behavior problems are also symptoms of allergic disorders, but links have not been proven.
Allergy testing may determine the cause of an allergy, but only for those conditions that have been associated with allergy. Allergy tests look for sensitization to substances that cause an allergy. This sensitization causes an antibody called IgE (immunoglobulin E) to form. The immune system uses antibodies to identify foreign invaders (like bacteria and viruses). The IgE antibody latches on to mast cells (one type of tissue cell) and circulating basophils (white blood cells). Mast cells are found in large numbers in body systems where allergic reactions are often apparent—around blood vessels, airways, gastrointestinal tract and skin. Allergy tests look for the presence of the IgE antibody in response to specific allergens.
A key point to remember about allergy testing is that it is used to confirm a suspicion of allergy. Pediatric allergists with Riley Hospital for Children at Indiana University Health will listen to your child’s history and perform a physical exam. Based on this information, your physician will then choose the appropriate allergy test 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:
- 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 only confirms or excludes specific sensitivity.
What are the types of pediatric allergy tests?
There are a number of different ways to test for allergies, including:
- Allergen challenge
- Skin test
- Epicutaneous (a skin prick test)
- Intradermal (injection applied under the skin’s surface)
- Blood tests
Allergen challenges are the most conclusive tests. During an allergen challenge, patients are exposed to the suspected allergen to see if symptoms occur. If the problem is due to allergy, the challenge will cause the same reaction and usually does not take a large amount of exposure to make symptoms appear. Allergen challenges can be dangerous because the potential reaction might be serious or even life threatening, so challenges are only performed under special, controlled conditions. Challenges usually are not done when the symptom is anaphylaxis (a severe and even life-threatening reaction to an allergen).
In our clinic at Riley Hospital for Children, allergen challenges are performed for foods, drugs, vaccines and local anesthetics. Most often, the child comes to us after experiencing a skin reaction after exposure to the potential allergen.
Due to the potential dangers of an allergen challenge, skin tests are another option used to help confirm an allergy diagnosis. We perform two types of skin tests in our clinic. The most frequently used test is the epicutaneous (skin prick) test. During this test, we use a plastic probe to place the suspected allergens under the top layer of skin. This test is usually performed on the patient’s back, and patients usually do feel a pinch from the probe. To keep the discomfort to a minimum, we apply the tests quickly. We wait 15 minutes and then “read” the results.
For all tests, we place two control tests: a positive control and a negative control. These control tests provide baselines to compare with the other tests. The positive control should cause a red mark with a lump, or wheal, in the center. With this type of reaction, we know that there are no medicines being used that could interfere with the test results. In other words, the positive control tells us it is a good day to test. The negative control makes sure that patients have no non-specific skin reactivity. If the negative control looks like the positive control, then there is non-specific reactivity. This means other results are potentially invalid.
A second type of skin test is the intradermal skin test in which allergens are injected into the skin with a very small needle. These are very sensitive tests. They tend to cause many false positive responses, are expensive, may result in systemic reactions and 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 since they frequently yield false positive results.
When compared to skin tests, blood tests for allergy are not as reliable. Blood tests are not as sensitive, specific or predictive as skin tests. The accuracy of results often depends on the lab performing the test. An allergist who orders allergy blood tests must have full confidence in the facility doing the test.
We use blood tests to follow the course of a food allergy. The blood test can provide a value for circulating IgE antibodies, and that value is associated with the risk of having a reaction. The values of circulating IgE to food decrease over time if the child is outgrowing the allergy.
The Choosing Wisely® campaign, an initiative to help physicians and patients reduce overuse of tests and procedures, reviewed blood tests used to diagnose allergy. The areas of concern were IgG (immunoglobulin G) allergy tests and IgE allergy panels.
An evidence-based review on these tests and panels showed no support for the use of IgG tests for allergy, particularly for IgG to foods. IgE blood panels include items not relevant to the child’s history; they are confusing, expensive and often interpreted incorrectly.
What do test results mean?
Allergy tests are scored in many ways. The important thing to note is that the score provides no real insight into how severe the allergy is in the patient. Severity of the allergy is based on the type of reaction caused by the allergen. For example, a child might have a low score on the test, but experience anaphylactic shock in reaction to the allergen. Another patient may have a high score on the test and experience hives. We use test results to determine only if there is or is not sensitivity to the allergen.
In addition, the value of the test is only 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.
Finally, an allergy test may result in a false positive, which means that the test shows sensitivity but the child does not experience any clinical reaction. Therefore, allergy tests are an important part of the diagnosis, but must be considered with many other things to confirm the diagnosis of an allergy.
Who can be tested?
Parents are often told that their child is too young to be tested, but this depends more on the type of allergy being tested. In children, the first allergies to cause reactions are food allergies. These show up as atopic dermatitis (skin irritation). This illness may appear after the first three months of life. An infant with dry, itchy, flaky skin who is more than three months old could be tested to help find out if a food is an allergic trigger for this problem.
On the other hand, testing very young children for pollen allergy is not necessary. 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 airborne allergens that would be more pertinent, such as cat or dog dander. These are more perennial allergens that can lead to sensitization in the young child.
How should I prepare my child for allergy testing?
Do not use any antihistamines before allergy skin testing. A good rule of thumb is to avoid them starting a week before the test.
What happens next if the test is positive?
The most effective form of allergy treatment is avoidance. Allergy test results can also be used to formulate allergen extract for immunotherapy or “allergy shots.”