Once a dog has been diagnosed with allergies (allergic dermatitis), the next step is to determine the specific cause and remove it. Dogs may develop allergies to a number of things, far too many to list here. The most common causes I see are dust, corn, wheat, wool and grass, but it really depends on where you live.
There are three different tests used to diagnose this condition.
1.) Intradermal tests (skin tests): This test assesses the allergy antibody IgE in the tissues. The patient’s side is clipped into a large square and minute amounts of the antigens commonly found where the dog lives are injected into the skin. The reaction at each site is compared to a control area. Although the test is very accurate, only a limited number of antigens maybe checked.
2.) Serum allergy tests: This measures IgE circulating in the blood. A blood sample is all that is required. The sample may be analyzed for many more antigens than the intradermal test. Besides the antigens common to all dogs, there are regional screens that check for antigens from specific plants and other antigens specific to the area. Although most of these panels offer food screens, the dermatologists I follow believe the results are not helpful. They recommend a 12 week food trial with a hypoallergenic diet instead.
Please note that with both the intradermal test and the serum test, medication such as steroids and antihistamines will interfere with the results by limiting the immune response. Patients should be off of these medications to promote accurate test results. Before withdrawing any medication, talk to your veterinarian as sudden withdrawal of steroids may trigger a life-threatening crisis.
Also, some antigens are seasonal. If your pet’s allergies are seasonal, perform the test during the time when your pet exhibits clinical signs.
3.) Hypoallergenic diets: The proteins in these diets are hydrolyzed into small fragments, unrecognizable by the immune system. During the 12 week food trial, the dog must not eat anything else beside the prescription food, this includes treats and flavored medicine.
Hillier, A., Allergy Testing: Uses and Interpretation (VET-93), Western Veterinary Conference 2004 Proceedings.