In most cases, there will be absolutely no evidence of cancer on routine blood work when a dog is diagnosed with cancer. If you find this surprising, you're not alone. My clients are constantly shocked when blood work was recently completed, interpreted as normal, yet their dog is diagnosed with cancer just a few weeks later.
Most cancers are diagnosed by detecting an abnormality on exam, x-rays or ultrasound, then performing diagnostics (tests) to determine why that abnormality is present. Let's dive into this a bit further...
If we find a skin mass on a routine exam, an aspirate (a sample of the mass using a needle) or biopsy (taking a piece of tissue from the mass) is needed to determine if the mass is cancerous or benign. If the mass is cancerous, we would not expect there to be any change in this patient's blood work.
If a patient is limping and radiographs (x-rays) of the leg show destruction of a portion of the bone concerning for a bone tumor, a bone aspirate is usually recommended to determine if cancer is present. Again, we would not expect there to be any evidence of cancer on routine blood work.
If a patient is experiencing weight loss and decreased appetite, we may recommend an abdominal ultrasound to assess all of the abdominal organs. Some organs may look normal and some organs may look abnormal (too big or too small, with nodules or masses that are inappropriate). If an abnormality is noted, an aspirate is needed to determine if cancer is present - a blood test would not be expected to give us a diagnosis.
What is interesting is that a patient may have a huge liver tumor but completely normal liver values. Conversely, there are times when routine blood work shows a mild elevation in liver values, prompting a veterinarian to perform a liver ultrasound and discover cancer. Medicine is definitely not black and white - veterinarians are constantly gathering information (from tests, their exam, how the patient feels, the history) to determine the best next step.
Similarly, when we perform blood work for a patient that has cancer and is undergoing chemotherapy, the blood work tells us how that patient's bone marrow (and other organs) is handling the chemo. In 90% of cases, we are not performing the blood work to tell us about the patient's cancer. In the vast majority of cases, we can only find out if the tumor is responding by (1) doing tests like x-rays and ultrasound to determine if cancer has spread, (2) performing a physical exam (if the tumor was external) to make sure the tumor hasn't returned or is smaller (whatever our goal/expectation is). Each situation will be different for each patient and for each type of cancer that we are treating, so consult your oncologist or vet if you have questions about this.
On the other hand, blood work can help diagnose cancers that involve the bone marrow and immune cells (ex. leukemia, multiple myeloma, lymphoma that involves the bone marrow) and those that may cause elevated ionized calcium (ex. certain lymphomas, certain anal sac adenocarcinomas).
If a patient has leukemia (which starts in the bone marrow), we will see changes on a routine CBC (complete blood count) - this is very helpful in making the diagnosis. We can also monitor our success in treating leukemia using the CBC (we would want to see fewer abnormal cells and more normal cells as treatment continued).
Routine blood work is essential and very helpful in screening for general organ health and for numerous diseases, but unfortunately, cancer is not usually one of them.
Remember that early detection is essential in successfully treating many types of cancers. If you notice a new lump or bump on your dog, have it aspirated when it is small, don't wait to see if it gets bigger. If your dog isn't feeling well, have tests done to find out why. If your current vet can't seem to figure it out, consider going to a specialist. They see uncommon diseases every day and are great problem solvers.
Have questions about this article? Reach out!
Dr. Lori Cesario
Board Certified Veterinary Oncologist
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