Allergy testing is performed to develop a specific therapy for the allergic animal.
There are two basic types of allergy testing. The most common is a blood test that checks for antigen-induced antibodies in the dog’s blood. There are two standard tests that are used to test the blood. The first is called a RAST test (radioallergosorbent). The other is an ELISA test (enzyme-linked immunosorbent assay). Both of these tests are very similar, but many practitioners feel that the ELISA test gives more accurate results than the RAST test.
The other type of testing is intradermal skin testing. In this test, a small amount of antigen is injected into the skin of the animal and after a short period of time, the area around the injection site is observed to determine if the animal is allergic to the agent.
Blood testing by RAST and ELISA
To perform either of these two tests to determine what substances an animal may be allergic to, a blood sample is drawn from a suspected atopic dog and sent into the lab. The blood is screened for a reaction to a broad range of allergens including pollens, dust, and molds that are common to the geographical area in which the dog lives. Tests for reactions against different foods and contact allergies such as cotton or nylon can also be done, although, the tests perform less accurately for these allergens. The testing generally takes several weeks.
Unfortunately, the RAST and ELISA tests have some inherent problems in their design. They were initially developed for humans and the circulating antibodies that they test for are different in dogs and cats than in people. The result is that there are a lot of false positives. In other words, the test results indicate that your animal is allergic to things that he is really not. For this reason, veterinary dermatologists may opt for intradermal skin testing. The RAST and ELISA tests, however, are used in special circumstances in which intradermal skin testing cannot be performed, such as:
The skin is too unhealthy to test either due to infection or severe inflammation.
Prolonged drug withdrawal for antihistamines and corticosteroids cannot be tolerated.
The dog is very young.
The dog is a show dog whose hair cannot be clipped.
Intradermal skin testing is unavailable.
The intradermal skin test (IDST) is negative, yet atopy is still suspected.
As a screening test to limit the number of allergens tested for in intradermal skin testing.
Of all of the blood tests performed, the ELISA is the most accurate, and if a blood test must be used then this is the one the author recommends performing.
Intradermal skin testing
Intradermal skin testing is the gold standard of allergy testing for atopy. The animal is sedated, and an area on his side is shaved down to the skin. On this shaved area a small amount of antigen is injected into the dog’s skin. This is done in a specific pattern and order so that if the dog shows a small raised reaction, the offending antigen can be identified. After a period of time (hours), the shaved area is examined to detect which antigens, if any, created a reaction. Reading the reactions and determining if they are large enough to create an allergic reaction in the dog is somewhat of an art. This is where having a veterinary dermatologist really pays off.
Successful identification and treatment of atopic dogs by this method is listed to be as high as 75%. This test works best if performed during the season when the allergies are at their worst and is not necessarily more expensive than blood testing. Animals to be tested must not have been treated with steroids or antihistamines for several weeks to months before testing. The exact time differs with the drug used and type of test performed.
The basics of all allergy tests
The blood and intradermal skin tests have some similarities:
These tests work best if performed during the season when the allergies are at their worst.
Once the allergens for the dog are identified, an appropriate immunotherapy is manufactured for that specific dog, and treatment can begin. After the offending antigens are identified, then a mixture of these antigens can be formulated into a hyposensitizing injection. Depending on the type of agents used, these injections will be given over a period of weeks to months until the dog or cat develops immunity to the agents. After initial protection, an occasional booster may have to be given.
Before any animal is tested for allergies, a complete diagnostic workup ruling out other potential causes of skin problems should be performed. Ectoparasites such as fleas or mites, fungal or yeast infections on the skin such as Malassezia, and chronic bacterial infections must be eliminated. In addition, the possibility of concurrent hypothyroidism must be ruled out.
It is often recommended that animals be placed on a homemade or hypoallergenic diet for 12 weeks before undergoing testing to ensure that a food allergy is not the cause. After the cause of the skin problem has been identified as atopy, then it is time to do the test.
We will take this time to warn pet owners that if they are not committed to the necessary follow-up treatment, which involves numerous injections over a long period of time, or if the veterinarian does not offer immunotherapy (hyposensitization) as a treatment option, then all of these tests may be a waste of time and money.