Correcting Misconceptions About Allergy Testing in Cats

Blood test or skin prick test? Neither.


People often wonder which is better for diagnosing allergies in a cat: skin prick tests or blood tests. The answer: neither one.

For one thing, prick tests are not performed on cats. Rather, sometimes cats with suspected allergies undergo what is called intradermal testing. The difference is that with a skin prick test, a physician will put a drop of the allergen (the offending substance) right on a person’s skin and then scratch it by pushing around the applicator a little bit to see if the skin flares. Intradermal tests done on cats, by contrast, put potential allergens under the skin with a needle and then look for reactions in the form of wheals — red, swollen marks. The cat has to be shaved over part of her body and also sedated.

For all that, it is not easy to assess the results of intradermal testing in felines. But a blood test does work similarly in cats and people, so it is more commonly performed. A bit of blood is drawn and then sent to the lab to check for antibodies to different allergens.

But neither blood tests nor intradermal tests are supposed to be used as diagnostic tools for determining what substances a cat is allergic to. Why not?

“A cat can be found to have results from those tests that technically read positive for an allergy to a substance, but she may not suffer an allergic reaction that includes itchiness or other symptoms,” says Tufts veterinary dermatologist Ramón Almela, DVM. “It just means your pet is sensitized or has simply been exposed to the allergen.”

What it comes down to is that because of a positive result, you can end up with a situation where you’re treating an allergy, but not the one that’s making the animal miserable. And maybe there’s no allergic reaction at all. Maybe the cat is itchy or uncomfortable because of a disease that has nothing to do with an allergy.

“It’s a challenging situation from a communications standpoint,” Dr. Almela says. “It’s very important to explain to clients why these allergen tests are not reliable for diagnosis and address misconceptions about them.”

The only way to properly diagnose an allergy

Unfortunately, there’s no simple test for figuring out what substance(s) a cat is allergic to and then treating from there. Diagnosing an allergy is often an indirect process that requires time and patience.

For instance, if a food allergy is suspected, the only way to find out for sure is by completely removing the ingredient(s) in question for an average of 6 to 8 weeks and feeding your pet a different food (called an elimination diet) during that time. If symptoms resolve, you then put your cat back on the old diet and see if she flares up again. If she does, then she likely has a food allergy.

It’s easier said than done, with no flavored medications allowed, no treats, nothing the kids drop on the floor, no flavored toothpaste during the trial. Any of those could contain the ingredient in question, or another ingredient that could cause symptoms and muddy the results.

If airborne allergens are suspected of causing the symptoms — pollen, dust, mold — the veterinarian has to conduct a thorough clinical examination and take a complete history to arrive at a diagnosis of exclusion. In other words, allergies are often discovered by ruling out all other possibilities for a cat’s symptoms. It’s the same for diagnosing an allergy to parasites, such as fleas. It’s frequently discovered that the uncomfortable symptoms aren’t an allergic reaction but, rather, a bacterial infection or a skin condition that’s causing the itchiness.

Are blood tests or intradermal tests ever called for?

You can very reasonably wonder, based on the proper way to diagnose allergies in a cat, whether blood or under-the-skin tests for allergens are ever appropriate. The answer is yes, sometimes — but increasingly infrequently.

It has to do with the treatment chosen once the doctor has determined that a particular allergy is in fact causing a cat to suffer symptoms. At this time, there are three treatment options to relieve allergies in cats, Dr. Almela says:

  • Apoquel — A drug given in pill form to block allergic itching
    at the source. (It’s off-label for
    cats but used widely, with
    good results.)
  • Cyclosporine — An immunosuppressant drug also given orally that suppresses the immune sys-tem and thereby blunts the allergic response. (This is probably the most often used treatment for allergies in cats.)
  • Immunotherapy – A treatment in which injections or under-the-tongue administration of small amounts of the offending allergen are given to desensitize a cat’s system and reduce the likelihood of an allergic reaction.

Only one of these treatments, immunotherapy, requires a blood test or an intradermal test. In such cases, once an allergy has been diagnosed, either of those tests can help a veterinarian determine which allergens should go into the injections other than the symptom-causing allergen itself.

“We don’t know exactly how immunotherapy works,” Dr. Almela comments, “but the hypothesis is that it ‘teaches’ the immune system to stay ‘calm’ in the allergens’ presence. Unfortunately, to see a benefit you might have to wait several months, even a year in some cases.”

Why would someone choose immunotherapy for their cat when the other medications tend to work much more quickly — sometimes as soon as the day of administration — and much more reliably?

“Actually, the trend is away from immunotherapy,” Dr. Almela says. “There will always be a place for it because it is the safest of the three treatments. Other treatments may be contraindicated for certain cats; there can be untoward side effects. But still, it’s not used as commonly as it once was.

“A couple of decades ago, immunotherapy was one of the only options out there. We didn’t have all the tools we have now, which work faster in a cat — and also better, with a success rate of about 80 percent as opposed to about 60 or 65 percent for immunotherapy. So what is happening now is that fewer clients are choosing immunotherapy for their cats. I used to be asked by clients to treat allergic cats with immunotherapy much more frequently even a decade ago. The upshot is that blood tests and intradermal tests in allergic cats for the purpose of formulating an immunotherapy treatment with specific amounts of various substances are becoming less and less commonplace.”

Bottom line

If your cat is diagnosed with an allergy and you decide to try immunotherapy — perhaps with input from the veterinarian that one or more of the other treatments might disagree with your pet — a blood test or an intradermal test to check for the presence of specific allergen antibodies is appropriate in order for the doctor to be able to mix the right combination of substances for the immunotherapy trial. But if you and the doctor are talking about those tests up front, even before it is known that your cat even has an allergy, you could be identifying a problem substance that is not actually causing allergic reactions in your pet — and wasting money in the process. 


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