It's important to recognize that multiple triggers can coexist, and that these triggers can overlap to push a patient over the itch threshold. To further complicate matters, secondary infections (e.g., Malassezia and/or Staphylococcus) can exacerbate these conditions, worsening clinical signs. While it's tempting to use an allergen panel as a diagnostic tool, the diagnosis of environmental allergies (atopic dermatitis) is one made by clinical acumen and exclusion of other causes of pruritus.

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Before pursuing allergy testing, a diagnosis of atopic dermatitis should be made by:

• Ruling out an adverse food reaction with a strict prescription-grade elimination diet for at least 8 weeks, followed by a challenge if symptoms are not seasonal or intermittent.
• Controlling ectoparasites with flea preventive where environmentally and geographically appropriate.
• Managing secondary infections with appropriate topical therapy based on cytology.

How to Use Allergen Testing

There is a trend in veterinary dermatology to replace the words "allergy testing" with "allergen testing." While this is a subtle change, it highlights the importance of how experts use this test. Veterinary dermatologists use allergy testing to select allergens for allergen-specific immunotherapy ("allergy shots" or sublingual drops). Immunotherapy is the only treatment for atopic dermatitis that may alter the course of the disease by the induction of immunological tolerance with a goal to reduce reliance on pharmaceutical interference over time (perhaps 6-12+ months). There is no one-size-fits-all approach to immunotherapy, and counseling pet owners regarding expectations and monitoring patient response are integral parts of achieving success.

What Allergen Testing Measures

Veterinary professionals can perform allergen testing using intradermal testing, serum testing, or a combination of both. Intradermal tests directly assess the ability of a substance to cause an allergic reaction (e.g. mast cell degranulation) in the organ of interest. When it is safe to sedate the pet (i.e., the animal is an appropriate candidate for sedation) and no medications are present that could alter test results—such as oral, injectable, or topical glucocorticoids or antihistamines—IDT may be preferred because it directly tests the organ of interest. Serum allergy testing offers a less invasive option for patients that are either poor candidates for sedation or those unable to go for a referral following diagnosis of atopy. Serum tests are typically ELISA tests that assess the presence of circulating IgE antibodies in the serum, which can correlate with a Type I hypersensitivity reaction. For example, in my practice, I may opt for a serum allergy panel rather than an intradermal test in cases involving an extremely brachycephalic dog or one that cannot be safely or ethically withdrawn from glucocorticoids.

Interpreting Results for Immunotherapy

Regardless of which test is used, it's important to understand that both intradermal and serum testing can have false positives and false negatives. Both can be affected by the timing of sampling, and neither is valuable in a patient without documented environmental allergies (atopic dermatitis). Unfortunately, there is no clinical value in allergy tests for the diagnosis or management of food allergies. The real magic of allergen testing happens in the interpretation of the test in light of the patient's history and with an understanding of the biology and seasonality of the relevant allergens of that particular geographical area. It's essential to understand which allergens in your area are significant (e.g., molds, weeds, grasses, trees, mites) and how that correlates with your patients' clinical signs and environmental exposures so that you can select the most relevant allergens to include in a particular patient's immunotherapy—and at times, determine which environmental modifications to consider.

For example, in Oklahoma City, where I practice, oak pollen tends to spike in the spring, grasses in summer, ragweed in early fall, and red cedar in late fall/early winter. Other allergens, such as dust and storage mites, may be present year-round. When I'm formulating a patient's immunotherapy, I take their history into account, and I only include that which is relevant into their immunotherapy. If a dog with a history of chewing its paws in August tests positive for ragweed pollen, I would be sure to include it in their immunotherapy. If a dog with no exposure to cats tests positive for cat dander, I would counsel that owner about cat avoidance, but would not include that in that pet's immunotherapy.

If you are in an area with lots of rain or flooding (e.g., the Pacific Northwest), allergens like molds will be more relevant to your patients than they would be for a patient in Southern California. Additionally, some serum allergy panels will test a broad selection of allergens, regardless of geographical location. It's important to know what to include and what to ignore. For example, if a dog in Oklahoma City comes to me with a positive test result for palm trees, I will disregard it.

This is a nuanced process that requires a high level of awareness of your environment, observation, conversation, and thoughtfulness. A good resource for evaluating relevant allergens in your area may be the social media of your local human allergist.

Summing It Up

Allergen testing is not a diagnostic test but rather a therapeutic tool used to design allergen-specific immunotherapy for a particular pet in a specific geographical region. Careful interpretation of any allergen test is crucial for achieving optimal outcomes with immunotherapy, and it is essential to have a thorough understanding of the pet's history of symptoms, relevant exposures, and the biology and seasonality of relevant allergens.

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