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The diagnosis of atopy is not made by allergy testing. It is a diagnosis based on history, consistent clinical signs and ruling out other causes of itchy skin and/or otitis externa. Occasionally, non-atopic dogs and cats can have positive allergy test results – this is why it is vitally important that the correct diagnosis has been made first, before resorting to these tests, and why cases are best dealt with by a veterinary dermatologist or someone with a specific interest in veterinary dermatology.
Once a diagnosis of atopy is made, allergy testing can be performed to discover what allergens are significant if we are to consider trying to reduce allergen exposure (may help reduce symptoms but unlikely to abolish them completely), but is done primarily to decide upon which allergens to include in treatment using allergen-specific immunotherapy (ASIT). Two methods of allergy testing can be used. Intradermal Skin Testing (IDST) is still considered the gold standard test amongst veterinary dermatologists and there is also the choice of serological allergy testing. These tests measure allergy in different ways – IDST measures allergen specific IgE bound to mast cells and the blood tests measures allergen specfic allergic IgE in the bloodstream. There are sometimes differences with the results of testing by these two different procedures, although the success rate of ASIT are similar. Many dermatologists are now doing both tests if finances allow, as there appears to be a slightly better response rate if the “positives” from both tests are included in the immunotherapy.
Intradermal Skin Testing – IDST
The test kit that I use contains 40 different indoor and outdoor allergens, including house dust mites, forage mites, grass, tree and weed pollens and moulds.
Before this test is performed animals should be taken off certain medications for various lengths of time as stated as below:-
* This may not be neccessary if the steroid hydrocortisone aceponate (Cortavance) has been used, where the current recommendation is 2 weeks.
A very small quantity (0.05 ml) of each of the allergens and the control solutions is injected into the skin at various points – histamine is the positive control and the negative control is the solution that the allergens are contained in. After 15-20 minutes, all the injection sites are assessed to see if there has been any swelling and redness of the skin ( erythematous wheals). If a dog or cat is allergic to a particular substance, there will be an inflammatory response in the skin where it is injected and this will be seen as a swelling The size and degree of erythema of the wheals is compared with that of the control solutions, and this allows the animal’s allergies to be determined. The reactions we see in cats are much more muted that those seen in dogs and it takes a trained eye to see them – sometimes a fluorescent dye is injected intravenously and the injection sites examined with an ultraviolet light to evaluate the responses.
Serological Allergy Testing
Blood samples can be taken to measure the amount of allergen specific antibody ( Allergen specific IgE) there is in the bloodstream. Blood testing, until a few years ago, was not considered very reliable as many symptomless, non-atopic dogs could give positive results. The more recent tests such as the Heska Allercept Test and the ARTU serological test are much more reliable. Results of ASIT based on theses tests are nearly comparable with those based on IDST. They have the advantage that, although it is still recommended that animals are off steroids for 4 weeks before testing, results can still be obtained from cases which are or steroids or have only recently come off them.
If an animal has both IDST and Serological Allergy testing performed, there can sometimes be a discrepancy with the results and some dermatologists now recommend doing both these tests together, as choosing allergens for ASIT from the postitive results of both tests may result in a better response rate. Also, I have had cases which were negative on IDST but positive on serological testing and vice-versa which have gone on to respond to immunotherapy. Results of the Artu serological test are expressed numerically and any allergen with a score over 150 is considered potentially significant.
There are a very small proportion of dogs which fit all the criteria for canine atopy, but fail to produce reactions either on IDST or have positive allergen specific IgE on blood testing. These individuals are said to be suffering from “atopic-like dermatitis” and unfortunately are not candidates for immunotherapy, although can respond to the other therapies given to atopic dogs.