RAST Testing in Cats

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 things. 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 or chew themselves until their skin is raw, leading to more obvious skin sores and secondary infections. Some cats can develop a condition called eosinophilic granuloma complex, which manifests as ulcers on their lips (called indolent or “rodent” ulcers), or red, raised plaques in their mouth or on their skin.

"The most common signs of allergic skin disease in cats are redness and itching."

Miliary dermatitis can also be seen, which is the formation of multiple tiny scabs under the fur. These signs can be seen with any type of allergic skin disease: food allergy, flea allergy, or atopy.

How will my veterinarian diagnose atopic dermatitis?

The first step in working with 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 (skin mites), 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).  Skin cytology may also be recommended to assess for secondary bacterial and yeast infections.

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". 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 trigger a reaction. These specific allergens can be used in hyposensitization treatment for your cat (allergen specific immunotherapy). Hyposensitization involves giving your cat a small quantity of the offending allergens in a series of injections or liquid given under the tongue, 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 allergy testing (IDAT) 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 sedated. Large areas of your cat’s coat must be shaved to allow the injection sites to be monitored for a reaction.

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.

You may read or hear about allergy testing offered on saliva or hair, but there is little evidence to support the validity of these tests and they are not recommended by most veterinarians.

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

In many cases, RAST testing can be performed while cats are receiving their regular allergy medications. This contrasts with intradermal testing, which typically requires that cats be off medication for a prolonged period of time before testing. This may not be possible if your cat is severely affected.

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?

While multiple companies may offer this testing, not all are equally as reliable.  Your veterinarian may consult with a local dermatologist, if possible, to see what company they recommend.  Some of these panels also include testing for food allergens, but the results are controversial and do not replace an elimination diet trial. 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, molds, 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.