How Veterinarians Diagnose Cancer in Pets

Routine blood tests don’t typically tell us enough about cancer affecting our pets. By: sandiegochurch

When our pets are sick, our mind often trips into worst-case-scenario mode. Despite all my training, I did this when one of my own cats went lame. My mind immediately screamed “He might have bone cancer!” rather than the more likely alternative that my cat was in a fight (which was the case).

Indeed, when a pet isn’t well, vets often run blood tests. When we say that test results are normal or negative, commonly the person will let out a breath and say, “So he hasn’t got cancer.”

Well, we don’t know that because routine blood panels don’t diagnose cancer. True, we can get clues, but that’s often as far as screening tests go.

Medical History

It starts with gathering information about the pet with a history and a physical exam, paying particular attention to:

As well as building a general picture of the animal’s health, this can suggest problems other than cancer that need investigating. The vet then uses this information to run specific tests and reach a diagnosis.

Screening Blood Tests

For many pets with cancer, a routine blood test comes back normal. Why is this?

Quite simply, the test isn’t looking for cancer; it measures how well the organs function, electrolyte levels, and the size and number of red and white cells.

OK, there are blood cancers such as leukemia or some lymphomas that shift the white cell count, but these are the exception rather than the rule.

Think of it like a car mechanic diagnosing why a vehicle drives rough. If he just looked at the engine and found no problem, it doesn’t mean the vehicle is fine — because the mechanic didn’t spot that a tire was flat. And in just the same way, cancers often have a local effect that doesn’t show up in general blood tests.

Ultrasounds and X-rays can help veterinarians rule out cancer in pets. By: John Donges

The Purpose of Screening Tests

The thirsty cat who’s losing weight needs blood tests so that common problems, such as overactive thyroids, diabetes or kidney disease, aren’t missed. When these don’t provide the answer, the suspicion of cancer rises.

Also, there are 2 examples where clues to cancer do show up in the blood. These are:

  • Blood cancers, where too many of 1 type of blood cell are produced.
  • Bone marrow cancer, where too few of certain blood cell types are produced.

My Pet Has a Lump

When there’s a skin lump present, taking a sample of cells for analysis is ideal. This could be just a few cells (through a needle) or the whole lump (by removing it). Common procedures include:

  • Fine needle aspirate: A few cells are sucked up via a needle and syringe. This is noninvasive, doesn’t require an anesthetic and gives a rapid diagnosis for some cancers.
  • Biopsy: This is a surgical procedure to remove a small tissue sample. This requires sedation and local anesthetic.
  • Excisional biopsy: This is where the surgeon removes the whole lump, which is sent for analysis.

Next Steps

When a pet is sick but there’s no external lump, the vet’s next step is imaging. With radiography, ultrasound or a CT or MRI scan, a clinician can see what’s going on inside.

Each method has advantages and disadvantages. For example:

  • X-rays: Good for looking at bones and air-filled structures, such as the lungs, but not so great for soft tissue, such as the gut
  • Ultrasound: Gives super pictures of organs such as the liver, kidneys, bowel, bladder and heart
  • MRI or CT scan: Super-detailed slice through sections of the body. Helpful for pre-surgical planning if the pet has a tumor.

In some cases, the vet will perform an exploratory operation to investigate what’s going on internally.

This internet-famous kitty got an outpouring of love when he was diagnosed with cancer:

A New Blood Test

In the U.S., there’s now a blood test that looks at markers released in the bloodstream when cells multiply too quickly, as with cancer. Ask your veterinarian about it.

Unfortunately, even this test isn’t perfect, because slow-growing cancers can be missed, and the test gives a “yes–no-maybe” answer rather than pinning down the type and location. So X-rays or an ultrasound scan will still be necessary if the result is positive.

And finally, cancer caught early is often treatable. If you’re concerned about your pet, then have your vet check them over sooner rather than later.


This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed May 26, 2017.

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