Blood Tests for Cancer Screening?
November 23, 2011

I recently received a request to talk about a specific topic - newly
available blood tests for cancer in pets. I didn't know very much about the
subject, but looking into it was fascinating. Here's what I found.

Blood tests that look for the presence of biomarkers (i.e., something that
indicates the presence of disease) associated with certain types of cancer
are now commercially available. Two companies offer these tests, and they
take somewhat different approaches. One measures the blood levels of
tyrosine kinase, an enzyme that can mutate and cause unregulated cell
growth, which is the hallmark of cancer. This test can be used to look for
<http://www.petmd.com/dog/conditions/cancer/c_dg_lymphoma> lymphoma in dogs
and cats and hemangiosarcoma in dogs. The other type of test looks at a how
certain proteins are expressed in a blood sample (i.e., proteomic
biomarkers) and can be used to evaluate dogs for lymphoma. While the two
types of tests are different, they have many of the same pros and cons, so
I'll address them together.

First of all, these tests are not really "cancer screens." They cannot tell
you whether or not your dog or cat has cancer or is cancer-free. They only
evaluate for the specific cancers, lymphoma and/or hemangiosarcoma.

Also, calling them a "screening test" might make them seem a bit more
powerful than they really are. According to the National Cancer Institute,
screening is "checking for disease when there are no symptoms," but the
companies that make these tests freely admit that they should be used
primarily when there is already a high level of suspicion that a pet has the
disease in question.

For example, a dog presents with blood in its abdomen and a mass on its
spleen. The tyrosine kinase blood test might be useful in determining
whether the patient has a hemangiosarcoma versus a hematoma or other benign
mass. Another scenario where testing might be useful is in differentiating
between inflammatory bowel disease and intestinal lymphoma without the need
for intestinal biopsies, either via surgery or endoscopy.

I would not recommend these tests to my clients who have pets without
clinical signs associated with lymphoma or hemangiosarcoma. Why? The blood
tests have a relatively high rate of false positive results, which means
that a large number of clients will be told that their pets might have
cancer when they really do not. This will bring about a lot of unnecessary
worry and will necessitate additional, expensive diagnostic testing before
coming to a definitive diagnosis of "no cancer."

So as I see it, these blood tests for lymphoma and hemangiosarcoma could be
beneficial in very specific situations, but they are not true "cancer
screening tests." Keep in mind also that they have not been widely used and
therefore may have some glitches that we are not yet aware of. The results
should be looked at as just one more piece of information that must be
analyzed in combination with a pet's history, physical exam, and the
findings of more established diagnostic tests.

If anybody else has a topic they'd like to learn more about, pass it on and
I'll see what I can do.


Dr. Jennifer Coates


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