Dog Allergy Testing: Blood vs Skin – Key Differences and Recommendations

When presented with a patient that has a history of seasonally inflamed, itchy skin that responds poorly to maintenance medication, most veterinarians immediately begin to suspect allergies.

After ruling out parasites, skin infection, and other causes of pruritus and inflammation, the next step is often to recommend allergy testing.

The benefits and limitations of allergy testing

Allergy testing can be very effective in identifying the underlying cause of atopic dermatitis, a condition that is typically caused by the inhalation of pollen, mold spores, dust, and other allergens. Allergy testing is not, however, recommended for the diagnosis of food allergies. (Diagnosis of food allergies requires a food trial with a hypoallergenic diet.)

It is also important to note that allergy testing cannot be used to determine whether or not a patient has atopic dermatitis. Atopic dermatitis must first be diagnosed based on history, clinical signs, and the exclusion of other skin diseases. Allergy testing is only beneficial in seeking the underlying cause of already-diagnosed atopy, in preparation for immunotherapy and other treatments designed to decrease reactions to specific allergens.

Allergy testing in dogs typically takes one of two forms: intradermal skin testing, or blood (serum) testing. Each method has its own unique advantages and disadvantages, from both a medical standpoint and a client standpoint.

Intradermal allergy testing: How it’s done

Intradermal allergy testing involves injecting small quantities of allergens into the patient’s skin. This procedure is typically performed under sedation or general anesthesia, following a strict monitoring protocol. Approximately 40-60 injections are given in a single area of the body (typically the lateral thorax/abdomen) with a small sample of each allergen injected just under the skin.

A “positive” response to an allergen appears as a visible swelling, which looks like a hive. This swelling forms at the injection site and is typically observed within 20 minutes of injection. If such a swelling does not occur, a patient is assumed to not be allergic to that specific allergen.

By examining the visible skin reactions at each injection site, a dermatologist can determine which allergens are triggering the patient’s atopic dermatitis.

Pros and cons

Intradermal allergy testing is considered “the gold standard” for the diagnosis of atopic dermatitis. When other allergy tests are evaluated, they are often compared to intradermal testing.

There are numerous cautions and restrictions for patients receiving intradermal allergy testing. Females should not be tested if pregnant or in heat, due to hormonal impacts on testing. Pets should be tested at the end of their allergy season (typically in the fall). No baths should be given in the five days before testing, and pets must be fasted the morning of the procedure (to allow for safe sedation).

Additionally, intradermal allergy testing cannot be performed if the patient has received the following medications:

• Long-acting injectable steroids within the previous 90 days
• Oral steroids within the previous 30 days
• Topical steroids (including ear or eye medication) within the previous 14 days
• Antihistamines within the previous 7 days
• Essential fatty acids within the previous 7 days

This may prove challenging for some patients, who revert to extreme pruritus when medications are discontinued.

Other limitations to intradermal allergy testing include the need for sedation or anesthesia. This may be contraindicated in patients with certain underlying medical conditions, or a concern for some pet owners who are reluctant to pursue sedation/anesthesia for their pet.

Additionally, all hair must be clipped from the test site. With the proper veterinary table and equipment, this becomes no more than a mild inconvenience for most patients, but it may present a significant disadvantage to a show dog or a dog that is otherwise judged on the basis of its appearance.

Finally, intradermal allergy testing requires an experienced dermatologist to administer the test and interpret the results. Interpreting the skin reactions present at each site can be subjective, requiring significant experience to distinguish normal reactions from allergic reactions.

As such, the need for a dermatologist can present a significant obstacle for clients in rural areas who do not have access to mobile veterinary care and must travel significant distances.