“Isn't there a blood test you can do that will tell you if it’s cancer or not?”
If I had a dollar for every time I’ve been asked that question, well, I’d have a lot of dollars.
If I could invent a test that I truly believed could answer the question with accurate, honest, and reliable results, I’d have a lot more dollars.
Routine lab work is a fundamental part of staging a pet’s cancer. When I order those tests, I am ensuring that my patient is systemically healthy and that there are no "warning signs" of trouble regarding things such as organ function or electrolyte status.
However, such tests rarely provide information about a pet’s cancer status. With few exceptions (e.g., a very high white blood cell count could indicate a pet has leukemia or an elevated blood calcium level could result from several different types of cancers), lab work will not accurately inform me whether or not a pet has cancer.
There’s a difference between performing a test because we are suspicious that a pet could have cancer, and performing a test in a healthy patient to rule in/out a predisposition to cancer or occult (hidden) cancer that has not yet manifested with any clinical signs.
The latter scenario describes what are known as screening tests. These are tests designed to survey large populations and to “weed out” those individuals with a particular disease from those who are truly healthy.
The precise objectives vary, but most screening tests are designed to quantitate the presence of “biomarkers.” Biomarkers are measurable indicators of particular biological states or conditions and can be used to detect, screen, diagnose, treat, and monitor disease.
There are several commercially available tests available that examine different biomarkers for both cats and dogs. When we consider screening tests for cancer, most frequently, assays measure serum levels of thymidine kinase (TK) and C-reactive protein (CRP). The utility of these markers is not well established but emphasis is often placed on their ability to detect what we refer to in the medical profession as minimal residual disease (MRD).
TK is a protein involved in DNA synthesis and is expressed in dividing cells. TK levels increase with increased rate of cellular proliferation. TK levels correlate to the proliferative activity of lymphoid cells (and less likely with proliferation of other kinds of tumor cells). Elevated TK levels are also associated with viral infections and inflammatory conditions.
Serum TK levels tend to be higher in dogs with cancer than in healthy dogs. However, there is a great amount of overlap in levels measured from healthy dogs, dogs with cancer, and dogs with other diseases. Meaning that even dogs previously diagnosed with cancer can have normal serum TK levels.
TK levels have also been measured in cats and a reference interval was established from clinically healthy cats, cats diagnosed with lymphoma, and cats with inflammatory gastrointestinal disease. Cats with lymphoma had significantly higher serum thymidine kinase activity than healthy cats or cats with inflammatory disease and cats with non-hematopoietic neoplasia.
CRP is the major acute phase protein produced in response to inflammation and cytokine release. Serum CRP levels correlate to the duration and severity of inflammatory response. Causes of inflammation are varied, and include infection, autoimmune disease, and cancer. Therefore, CRP is considered a sensitive marker for inflammation, but unfortunately, it is relatively non-specific as to the nature of the inflammation it represents.
In dogs, CRP is elevated in at least some kinds of cancer, and serum levels are generally elevated in dogs with cancer compared to healthy dogs. As with TK, there is significant overlap between these two groups, and some dogs with cancer have normal serum CRP while some healthy patients have elevated serum CRP.
Dogs with lymphoma who are in remission, with only microscopically detectable cancer cells in their bodies, generally have lower CRP than dogs with measurable lymphoma. This places potential value on serum CRP levels as a marker for remission status and relapse of disease.
Additional research is necessary to determine the value of measuring parameters such as CRP or TK before veterinarians can routinely recommend these screening tests for every patient. Additionally, doctors must cautiously interpret the results of these tests, as information regarding the benefits and complications of instituting treatment at an earlier stage are unknown.
Lastly, if we are to consider implementing such tests, I suggest that owners should begin testing their pets at the earliest possible age, and test consistently throughout their lives, in order to establish the most adequate control values with which to compare to.
I absolutely understand why owners would wish for a simple lab test that could reassure them their dogs and cats are as inwardly healthy as they appear on the outside. I also understand the importance of early detection of disease and how this could lead to a more favorable long-term outcome for a pet.
However, I cannot ignore the sizeable gap of evidence-based information between these two poles regarding the utility of screening tests for cancer in companion animals that needs to be filled before veterinarians should be routinely recommend such diagnostics for their patients.
Dr. Joanne Intile
Image: Dmitry Kalinovsky / Shutterstock