Allergy testing is one of the most common reasons cats and dogs visit a veterinary dermatologist. When considering allergy testing for a beloved pet, there are several factors to consider:
- Do I need to perform allergy testing to diagnose my pet with allergies?
- Is allergy testing accurate? If so, what tests are best?
- Is my dog or cat a good candidate for allergy testing? If so, which test would be optimal?
- Once allergy testing is performed, what do I do with all that information?
Is allergy testing needed to diagnose a pet with allergies?
No. In fact, diagnosing allergies requires a clinical diagnosis, which occurs when a pet has a medical history and clinical lesions typical of allergies, and other causes of these clinical signs have been ruled out. Allergy testing is recommended only when a diagnosis of allergies has been performed. This is because allergy testing can detect positives even in animals who do not have allergies—this patient could be generating allergen-specific antibodies, but these antibodies are not leading to inflammation or itch. A classic example in our geographic area would be a patient with sarcoptic mange, a disease caused by an itchy contagious mite endemic on our fox population. Dogs with sarcoptic mange (“scabies”) are tremendously itchy and are commonly misdiagnosed with allergies. Allergy testing on these patients might well reveal some positives to tree pollens or weed pollens, but once the sarcoptic mange is treated, these patients usually no longer show clinical signs of allergic skin disease. Therefore, it is strongly recommended to have a solid clinical diagnosis of allergic skin disease before performing allergy testing.
Is allergy testing accurate? If so, what tests are best?
It is crucial to understand that allergy testing is currently only accurate for environmental allergies—allergens to pollens, trees, weeds, molds, pollens, and house dust mites, for example. It is not currently accurate at detecting what food allergens are afflicting the pet. This is true regardless of what type of food allergy test is selected—serum (blood), saliva, or patch testing. Positive values on a food allergy test simply mean that the pet has been exposed to that allergen or something similar. The only value in performing food allergy testing is in negative values—they usually can accurately predict what the patient is NOT allergic to. Currently, the only accurate way to diagnose food allergies in dogs and cats is to perform a strict elimination diet trial using either a prescription hypoallergenic diet or a home-cooked diet.
For environmental allergies, allergy testing can be accurate and helpful. There are two forms of testing that can be considered: intradermal allergy testing and serum (blood) allergy testing.
- Intradermal Allergy Testing: This is considered the gold standard method of identifying environmental allergens in dogs and cats (and horses). This test requires light, reversible sedation to prevent stress and anxiety. It involves injecting allergens (danders, house dust mites allergens, pollens—trees, weeds, grasses, molds) directly into the skin. Then, the size of the allergic reaction (wheal) is measured in regard to its firmness, redness, and size, and compared to a positive control (pure histamine) and negative control (saline). This test identifies allergen-specific antibodies on mast cells inside the relevant organ, the skin. This distinctive “pro” must be weighed against the potential “con” inherent with the risk of sedation and the very slight risk of an allergic reaction occurring during the procedure.
- Serum (blood) allergy testing: This test is easy to perform, as it is a simple blood draw. It aims to detect allergen-specific immunoglobulin E (IgE) to grasses, tree pollens, weed pollens, molds, and house dust mites, as well as some insects. The most important consideration with this test is the laboratory the sample is submitted to, as there are very important distinctions between laboratories regarding their accuracy. For example, it is crucial that the laboratory use CCD blockers. CCDs are Cross-reactive Carbohydrate Determinants that occur on many plant allergens. Animals can develop antibodies to CCDs; because these molecules can be present across many different allergens, many false positives can be detected. By blocking the CCDs, this false positivity is prevented.
Is my dog or cat a good candidate for allergy testing? And if so, which test?
Again, the most important criterion for allergy testing is that the patient has been appropriately diagnosed with environmental allergies. After that distinction has been made, there are still a few important factors to consider:
- Age of pet: It is important that the pet has been exposed to at least a year of allergens in its current environment. This means that testing should be delayed until the patient is over 1 year of age, and that individuals who have moved should experience their new environment for 1 year prior to testing.
- Medications: Patients on anti-histamines and steroids will need a wash-out period of several weeks prior to intradermal allergy testing, as these medications suppress the test. It is thought that serum allergy testing will still be reasonably accurate when on these medications, though long-term steroid exposure can also suppress this test as well. Atopica, Cytopoint, and Apoquel have been demonstrated to have little effect on either kind of allergy testing.
- Current skin condition: Individuals with significant skin infections are likely not good candidates for intradermal allergy testing. Serum allergy testing is the best option for these patients, or the intradermal test can be scheduled for a different visit.
- Show animals: Intradermal allergy testing requires shaving a small area of hair over the side of the patient, about the size of a postcard. This may mean that show dogs—or individuals who don’t have many active hair growth cycles per year, such as Nordic breeds like huskies or malamutes—may be better candidates for serum allergy testing.
- Concurrent health conditions: Individuals for whom sedation is contra-indicated—patients with heart conditions, for example, or breeds that can have difficulties safely tolerating anesthesia (French and English bulldogs, pugs)—are likely better candidates for serum allergy testing.
- Seasonally afflicted animals: While there is good evidence that non-seasonal patients can be accurately tested at any point in the year, patients whose allergies are worse in certain seasons will likely have more revealing allergy testing if it’s performed in one of their “bad” seasons.
- Should we consider both tests? There is some evidence that performing both intradermal and serum allergy tests can be most helpful when identifying allergens for allergen specific immunotherapy (more below). I often perform whichever test initially makes sense for the patient, then decide to do the other test based on those results.
Once allergy testing is performed, what do I do with all that information?
There are 3 main reasons to perform allergy testing:
- Predictability: By identifying major allergens and understanding what time of year those allergens are present, we can predict when the patient will need more intervention, and we can act proactively to decrease incidence of allergy flare-ups.
- Avoidance: When we know what the patient is allergic to, we can come up with reasonable avoidance tips to help decrease exposure to the allegens. It’s important to me that we let dogs be dogs and cats be cats, so these are low-key, common-sense tidbits that we can do in our everyday life that don’t negatively affect quality of life.
- Immunotherapy: To me, this is the most important reason to do allergy testing. When we identify what a patient is allergic to, this opens up a treatment modality called allergen specific immunotherapy (ASIT). While our other therapies involve giving medications to suppress clinical signs, ASIT aims to treat the underlying cause of the skin allergies. It involves administering, either via injections or orally, small amounts of the patient’s allergens on a regular basis, to gradually build tolerance to the allergens. By training the immune system to ignore the allergens, ASIT can actually alter the course of the disease—in about 70% of dogs and cats, less or no medication is needed long-term with ASIT. It’s important to understand that, if effective, ASIT is most often continued long-term, because discontinuing this therapy can often lead to relapses.
