What a Complete Blood Count (CBC) Can Tell Us About Your Pet's Health

By Joanne Lynn Intile, DVM, MS, DACVIM on Jun. 17, 2015

We’ve all been there.

Staring in anticipation at the sharp and shiny needle, poised above our arm, ready to pierce tender skin and withdraw a sample of our blood for some purpose related to our well being.

Bloodwork is a fairly diagnostic test prescribed by doctors. It’s performed to ensure we’re as healthy on the inside as we appear on the outside, or to monitor previously diagnosed medical conditions. The same is true for companion animals, and veterinarians utilize the same tests that are used in people to help us better assess our patients' physical status.

The most common blood tests I recommend are a complete blood count (CBC) and a serum chemistry panel. Each test provides me with very different but remarkably complementary information.

A CBC measures a patient’s white blood cell count, red blood cell count, platelet count, and usually provides some information regarding the size and/or shape of the red and white blood cells.

A chemistry panel provides values related to organ function (e.g., liver and kidney), as well as electrolyte levels and other important enzymes that can be measured in the bloodstream.

I am fortunate to have the option of having lab work performed directly in the hospital where I work. This means results are usually available within a few short minutes of a pet arriving for an appointment, and I can make important decisions regarding their treatment plan right away.

In less urgent situations, I can send blood samples to a larger laboratory located off-site and the results are typically available later that same day or the following day.

There are actually a "variety" of CBC and chemistry panels I can order, each offering slightly different information depending on what I am looking to measure and what information I am hoping to learn.

For example, I can send blood out for a “routine CBC,” or I can order a “CBC with a pathology review.”

The former provides strictly numerical values related to the counts of cells in the sample obtained by a diagnostic machine.

For the latter, a clinical pathologist will actually evaluate a sample of the blood under a microscope to confirm that the counts provided by the machine are accurate and to also determine if there are any abnormal cells present, damage to the cells consistent with certain toxins or poisons, or even evidence of parasites that can live in the blood stream.

I can order a full chemistry panel, which will give me over 25 different values, or I can just order a “renal panel” to tell me information about a pet’s kidneys.

Despite the wealth of information bloodwork can tell me, rarely do the results provide information about whether a patient has cancer or if their cancer has spread in their body. This is a difficult point for many owners, who wonder why I want to have bloodwork performed so often when it “doesn’t really tell [me] anything.”

I explain to owners that CBC and chemistry panels assure me that my patient’s body is handling the prescribed treatment plan without complication. I would much rather pick up on a mild anemia (lowered red blood cell count) or slightly elevated kidney value that occurs secondary to chemotherapy prior to a pet vomiting uncontrollably from organ failure or collapsing from weakness related to blood loss.

Each parameter measured on bloodwork is associated with a particular reference range, which encompasses a series of values between a specified low-end measurement and a high-end measurement. The specifics will vary, but in general, the reference range of any particular value encompasses the average of values obtained from apparently healthy animals, plus or minus some predetermined number of standard deviations.

Veterinarians are taught how to interpret lab work very early on in their curriculum. We learn what each of the dozens of abbreviations stand for, which body system or systems they are associated with, and what things we should be thinking about when the values are outside of the “normal” reference range.

What we also learn, in a surprising number of cases, is how to dismiss a value that falls either too low or too high on the scale as something we shouldn’t be concerned about.

Is the patient’s albumin level too high? Don’t fret, it just means they are dehydrated.

The lipase is low? Meh — that means nothing.

Say the cholesterol is 100 units over the high end of normal. Despite how hard your own MD probably comes down on you about your trying to keep your own blood cholesterol levels below a certain value, veterinarians don’t pay too much attention to it in an otherwise happy pet. It probably just means they weren’t fasted before the sample was taken.

When I talk to an owner about lab work results, some are happy to hear that my interpretation of things is “normal.” Others pore over each and every detail with the diagnostic acumen of a forensic investigator. They focus so much on the numbers that they miss the bigger picture of what is truly going on with their pet’s health.

Labwork is a very important part of my patient’s medical record and I’m happy to spend time explaining this to owners so they feel empowered about their pet’s care. I also want them to understand the limitations of what these tests tell us so that everyone’s expectations are the same. The amount of information garnered from that simple syringe and needle is truly remarkable.

In a future article, I will discuss the pros and cons of several commercially available blood tests designed to aid in the diagnosis and treatment of cancer in animals.