Diagnosing Your Allergies

Since allergies play such a significant role in lost productivity, increased healthcare costs, and simply making your life miserable, it is important to determine which allergens are responsible for specific diseases, so the proper medical decisions can be made.

 Allergy testing has serious limitations, and the diagnosis of an allergy to a specific allergen cannot be made on the basis of testing alone. Your history is just as critical to the diagnosis. The basic conventional forms of allergy testing include percutaneous testing, intradermal testing, in vitro antibody testing, and delayed hypersensitivity testing.

Percutaneous Testing

This is the skin prick testing most of us are familiar with. The basic mechanism behind skin testing is the interaction of the injected allergen with specific IgE antibodies on the surface of your skin mast cells.

This injection will trigger the release of histamine and the formation of a wheal and flare at the site. A wheal is the swelling you see; a flare is the redness. This reaction usually will occur within fifteen minutes after the allergen is introduced. This test remains the primary diagnostic procedure to determine the cause of allergies in this country.

Most practitioners perform this test on the back of the forearm, the upper arm, or the upper back. The upper back is by far the most sensitive but is not used as often. Certain guidelines should be followed to ensure that the test is done properly. For example, each allergen must be a certain distance apart, never done near the wrist or the elbow, and skin testing should never be performed on sites of active skin flare-ups such as dermatitis or hives.

Your doctor should use both positive and negative controls. A negative control tests the diluent that the allergen is in, to make sure you are not allergic to that rather than to the allergen. A positive control is usually histamine itself, to ensure that your body’s immune system is giving an adequate response.

The prick test can be performed in patients as young as one month of age, although this is quite rarely done. Allergen skin reactions start to decline in adults after one’s twenties, due to decreased skin reactivity to histamine and lower IgE levels. Therefore, if you are older than this when this test is done, you may get many false negative results.

This test is also limited because it measures only a clinically immediate IgE hypersensitivity. If you do not have an IgE-mediated allergy, the test will be negative, and your doctor will tell you that you are not allergic to a certain substance.

The test is also dependent on the person performing the test. Such factors as the exact amount of allergen used, the depth and force of the needle, the duration of force, the angle of application, and the stability of the allergen extracts are all variables that can cloud interpretation of the test.

Use of antihistamines should be stopped twenty-four to seventytwo hours prior to taking these tests; use of tricyclic antidepressants and benzodiazepines (Valium and similar substances) need to be stopped for seven to fourteen days beforehand; use of systemic corticosteroids and topical steroids should be stopped up to three weeks prior to any testing.

It is believed that nonsteroidal medications such as ibuprofen do not interfere, but I always advise my patients to stop use of these as well. You do not want anything to interfere with the accuracy of your test results. It is better to get a proper test result than one that does not give you the correct information.

Intradermal Testing

This is used when skin prick testing is not deemed sensitive enough to detect the cause of an allergic reaction. This is usually what happens when a patient tests negative on a prick test but has a strong clinical history of symptoms triggered by exposure to a specific allergen. This should also be used in patients for whom skin prick testing is not valid, as in anyone over thirty.

However, skin prick testing usually is done first, to avoid a systemic allergic reaction, which may be quite serious. Intradermal testing is performed through injection of an allergen extract that is diluted a hundred to a thousand times of what would be given in a skin prick test.

It is injected into the back of the forearm or on the upper arm. Swelling occurs immediately; changes in the size of the swelling and the redness are measured after twenty minutes.

This test also has limitations because small positive reactions may actually not be reactions, and positive and negative controls must be used so the test is interpreted properly. Despite their many drawbacks, percutaneous testing and interdermal testing are the most widely used conventional allergy tests.

In Vitro Antibody Testing

The first test of this kind was the RAST (radioallergosorbent test). It is a simple blood test that measures the amount of IgE that binds to a specific allergen versus the amount of IgE that doesn’t. This test can be used in patients for whom skin testing cannot be performed, such as those who cannot stop taking their medications, those with severe skin conditions, and those who have near-fatal reactions to certain offending substances.

The main disadvantage of this type of test is that there is no uniform method for reporting results, making separate tests not comparable to each other and difficult to use in clinical practice with any certainty. I hardly ever recommend this test because of its lack of useful information.

Delayed Hypersensitivity Testing

Whereas skin prick tests measure IgE immediate hypersensitivity responses, delayed hypersensitivity testing uses patches to measure type 4 delayed hypersensitivity. A clinical example of this involves contact dermatitis.

The antigens that cause this type of reaction are found in cosmetics, jewelry, household cleansers, and similar products, as opposed to pollens and foods. This test is performed by applying various materials to an absorbent pad, which is then placed on your skin, usually your back.

The site is then checked at forty-eight and at seventy-two hours after application. A positive response is characterized by redness and swelling. Most of us have taken a test like this such as for tuberculosis—a PPD.