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RAST Testing in Cats

By Catherine Barnette, DVM

Care & Wellness, Pet Services

What is RAST testing?

A RAST test, or radioallergosorbent test, is a blood test that can be used to determine whether a cat is allergic to certain antigens. This test is often performed as part of the workup for atopy (a skin manifestation of inhalant allergy) in cats.

Allergy testing allows you and your veterinarian to determine which allergens may be triggering your cat’s atopy, allowing you to formulate a plan to address these allergies.

What is atopic dermatitis?

There are three common causes of allergic skin disease in cats:

  • Food allergy: an allergy to the proteins found in food
  • Flea allergy: an allergy to proteins in flea saliva
  • Atopy: an allergy to inhaled environmental allergens

Atopy is skin reaction to inhaled irritants, and a common cause of seasonal allergic skin disease in cats.

What are the signs of atopy?

The most common signs of allergic skin disease in cats are redness and itching. In some cases, cats may scratch themselves until their skin is raw, leading to more obvious skin lesions and secondary skin infections. These signs can be seen with any type of allergic skin disease: food allergy, flea allergy, or atopy. See the handout “Inhalant Allergies (Atopy) in Cats” for more information on this allergy.

How will my veterinarian diagnose atopic dermatitis?

The first step in working up a cat with suspected allergic skin disease is to rule out non-allergic skin disease. Your veterinarian may perform a skin scrape to rule out mange, a fungal culture to rule out ringworm, and other tests based on your cat’s clinical signs and history. If these tests are negative, and your cat’s clinical signs and history fit, your veterinarian will diagnose your cat with allergic dermatitis (allergic skin inflammation).

"Once your pet has been diagnosed with allergic dermatitis, further testing is needed to determine the cause of your cat’s allergies."

Once your pet has been diagnosed with allergic dermatitis, further testing is needed to determine the cause of your cat’s allergies. Atopy is a diagnosis of exclusion, meaning that your veterinarian must first definitively rule out food allergy and flea allergy. Improved flea control may be used to rule out flea allergy, while a food trial must be used to rule out food allergy. If no improvement is seen with either of these treatments, your vet will diagnose your cat with atopy. For information on flea control, see the handout "Flea Control in Cats" and if your veterinarian recommends a food trial, see the handout "Food Allergies in Cats" for information on completing a food trial.

What role does allergy testing play in addressing atopy?

While some cases of atopy can be managed with symptomatic treatments, it is often helpful to determine which allergens are triggering a reaction.

"Although it is usually impossible to limit a cat’s exposure to allergens knowing the specific allergens causing an allergic response can be used to administer hyposensitization treatment to your cat."

Although it is usually impossible to limit a cat’s exposure to allergens knowing the specific allergens causing an allergic response can be used to administer hyposensitization treatment to your cat. Hyposensitization involves giving your cat a small quantity of the offending allergens (as injections or by mouth) every 1-4 weeks, in order to build up immune tolerance.

Approximately 60-80% of cats experience improvement with hyposensitization, although it may take some time to see benefits and some cats require lifelong therapy.

How is allergy testing performed in cats?

Allergy testing in cats can be performed using two techniques: intradermal skin testing or RAST testing. Each method has its own pros and cons.

While intradermal skin testing has long been regarded as the 'gold standard' in determining the cause of atopic dermatitis, it is only performed by veterinary dermatologists. Clients seeking intradermal testing for their cats must travel to a dermatologist which may not be available to all pet owners (not all areas have dermatologists and the cost can be high). Additionally, intradermal skin testing requires your cat to be anesthetized. Large areas of your cat’s coat must be shaved, in order to allow the injection sites to be monitored for a reaction.

"RAST testing requires only a single blood sample."

In contrast, RAST testing requires only a single blood sample. This blood sample can be drawn by your regular veterinarian during a routine wellness visit. The blood sample is submitted to a reference laboratory for analysis. No sedation or anesthesia is needed, and your cat will not need to have hair shaved for this test.

Will my cat have to go off her allergy medications prior to RAST testing?

In many cases, RAST testing can be performed while cats are receiving their regular allergy medications. This is in contrast to intradermal testing, which typically requires that cats be off medication for a prolonged period of time before testing.

Are there drawbacks to RAST testing, compared to intradermal testing?

In theory, yes. Historically, RAST testing has been associated with a higher rate of false positive test results, or positive results that cannot be confirmed with intradermal skin testing.

In a practical sense, however, evidence suggests that most cats treated with hyposensitization based on RAST results still experience a significant improvement in their skin disease.

Is there anything else that I should know about RAST testing?

It is important to understand that RAST testing is typically only worthwhile if you plan to pursue hyposensitization for your cat. It cannot be used to diagnose atopic dermatitis, only to identify the underlying cause of already-diagnosed atopic dermatitis.

In most cases, the allergens identified on RAST testing represent a wide variety of weeds, trees, grasses, and other allergens. Avoidance of these substances is typically not very practical for most cats and clients; therefore, the true benefit of this test lies in guiding the formulation of hyposensitization therapy.

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