Is the Allergy RAST Serum Test Really Necessary for Your Pet?

Skin & CoatQ&AMeongsiljang Veterinary Advisory Board

RAST is a blood test that measures antibodies against specific allergens. It is used as an adjunct during the preparation phase for immunotherapy after an atopic dermatitis diagnosis.

What Is the Allergy RAST Serum Test?

A veterinarian drawing a blood sample from a puppy's front leg.

The allergy RAST serum test is a serological test that measures the amount of antibodies (IgE) in the blood that react to specific allergens. The most important thing to understand is that this test alone cannot diagnose atopic dermatitis. Veterinary dermatology textbooks recommend using serum tests like RAST only as an adjunctive test to select allergens for immunotherapy in pets already diagnosed with atopic dermatitis based on clinical signs. The proper sequence is to first rule out parasites, bacterial infections, and food allergies.

How Does RAST Produce Results?

RAST stands for Radioallergosorbent Test, a technique originally developed to measure allergen-specific IgE antibodies in serum using radioactive isotopes. In veterinary medicine, this type of serological allergy testing is collectively referred to as ASIS (Allergen-Specific IgE Serology Testing), though many pet owners still commonly call it a "RAST test." Its greatest advantage is that a single blood draw allows us to simultaneously assess serum IgE responses to a wide range of environmental allergens, such as house dust mites, pollen, mold, and insect proteins.

RAST Serum Test vs. Intradermal Allergy Test Comparison

Item	RAST Serum Test	Intradermal (Skin) Allergy Test
Test method	Single blood draw	Shave the hair and inject allergens into the skin
Anesthesia required	Not required	Often requires sedation/anesthesia
Medication withdrawal period	Relatively short	Requires long-term withdrawal of steroids/antihistamines
Accuracy (vs. gold standard)	Supplementary	Standard by veterinary dermatology specialist criteria
Possibility of false negatives/positives	Relatively high	Low, but requires interpretation experience
Common situations for use	When specialist access is difficult / preparing for immunotherapy	Designing immunotherapy after a formal atopy diagnosis

Using either test on a pet that does not have atopy can lead to misleading results.

When Should You Consider a RAST Test?

The RAST serum test is most meaningful when all of the following conditions are met: - Clinical diagnosis confirmed: A veterinarian has diagnosed atopic dermatitis based on criteria such as the Favrot criteria. - Differential diagnoses ruled out: Parasites like fleas and mites, bacterial or Malassezia infections, and food allergies have been excluded. - Immunotherapy is being considered: The test is needed to identify specific allergens before starting allergen-specific immunotherapy (via injection or sublingual drops). It is not appropriate to use this test simply out of curiosity about what might be causing your pet’s persistent itching.

A veterinarian explaining allergy test results to a pet owner.

Key Points to Confirm Before RAST Testing

Before proceeding with testing, it is essential to thoroughly rule out underlying causes. Specifically, we need to confirm: 1) whether flea and tick preventatives are being used correctly; 2) whether an elimination diet trial using hydrolyzed food or similar diets has been conducted for an adequate duration; and 3) whether there are any concurrent bacterial or yeast (Malassezia) skin infections, verified through skin cytology. Allergy testing should only be considered after these potential causes—parasites, bacterial infections, and food allergies—have been properly excluded. Additionally, certain medications can interfere with serum IgE test results. Therefore, do not start, stop, or change any medications without consulting your veterinarian, who will determine the optimal timing for testing based on the type and duration of current medications.

How to Read the Test Results

RAST results are typically presented in a table showing the numerical value and reaction level (low, moderate, high) for each allergen. There are three common points of confusion to keep in mind: - A positive reaction does not confirm the cause: A high value does not definitively mean that specific allergen is causing your pet’s symptoms. - Higher numbers do not equal greater severity: A value that is twice as high does not mean the symptoms are twice as severe. - Interpret results alongside clinical signs: The test is only meaningful when the results are correlated with your pet’s medical history, specifically noting when and where they are scratching.

A desk with an allergy test result sheet and a stethoscope on it

Management Points After RAST

Once the test results are in, management typically follows two main approaches. - Environmental management: Reduce exposure to avoidable allergens such as house dust mites and pollen by frequently washing bedding, using air purifiers, and adjusting walk times. - Allergen-specific immunotherapy: This involves gradually desensitizing the body by regularly administering very small amounts of the identified primary allergens. Immunotherapy does not produce immediate results, and the speed of response varies by individual, so consistent long-term treatment under veterinary guidance is essential. Stopping treatment arbitrarily midway may require restarting the entire process from the beginning.

Points for Cat and Small Breed Owners to Note

In cats, neither intradermal testing (IDT) nor serum IgE testing (ASIS) is sufficiently standardized, and both can yield false-negative or false-positive results, so findings should be interpreted conservatively. In fact, studies have shown that serum testing cannot reliably distinguish cats with allergic skin disease from healthy cats or those with other pruritic conditions. For cats that scratch excessively or engage in compulsive hair pulling, flea control and dietary elimination trials should take precedence over allergy testing. Although certain breeds are known to be more prone to atopic dermatitis, test results should never be equated with treatment; instead, lifelong management—including skin barrier protection, bathing routines, and nutritional care—is essential.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

Can a single RAST test identify all the causes of allergies?

Not exactly. RAST measures antibodies in the blood, so it doesn't always perfectly reflect what's actually happening in the skin. To get closer to the true cause, we need to consider the clinical findings, dietary trials, and intradermal skin test results together.

I found an online test for hair and saliva allergies. Is it safe to try?

I don’t recommend it. Atopic dermatitis is primarily diagnosed clinically, and even allergy testing is not advised as a standalone diagnostic tool. Tests using hair or saliva samples have not been proven diagnostically valid, so they may only increase owner anxiety and lead to misguided treatment decisions. It’s best to consult your veterinarian and follow proper diagnostic procedures.

What should I do with the allergy medication my pet was taking before the test?

Please do not stop any medications on your own; be sure to consult your veterinarian. Some medications can affect serum IgE test results, so depending on the type of drug, it may be necessary to discontinue it before testing, while in other cases it is not. Your veterinarian will review the type and duration of any medications your pet is currently taking to determine the appropriate timing for the test.

Can food allergies also be confirmed with a RAST test?

RAST results are difficult to use as the basis for diagnosing food allergies. The standard approach is to confirm food allergies through an elimination diet trial of sufficient duration using hydrolyzed diets or similar specialized foods. Serum tests should only be used as supplementary reference information.

If all the RAST test results came back as "low," does that mean there's no allergy?

Not necessarily. There are cases of false negatives, where clinical symptoms are clear but test results come back low, and there are also non-allergic causes of itching. It’s best to discuss with your veterinarian to reassess the approach.