As discussed in Part 1 of this series, chemotherapy can really affect (destroy) white blood cells, particularly the infection-fighters called neutrophils. Let’s look more closely at what tests oncologists use for white blood cell counts.
Why So Many Tests??
Oncologists monitor the white blood cell count closely and often! Remember, this is important because this is one of the main ways we decide the dose to use in our next treatment.
Neutrophils, the white blood cell we are really concerned about, only lives 18 to 24 hours in the bloodstream. That means that yesterday’s test is no longer accurate. And a complete blood count (CBC) from a few days ago or last week is truly not helpful. The neutrophil count changes daily. We simply have to keep running those tests.
Test White Blood Counts Before Chemo …
The white blood cell count needs to be checked the day of chemotherapy treatment with a CBC, which is a very routine test. The veterinary tech will take some blood from your dog, and it will be analyzed by special equipment. Most hospitals have the ability to run this test right away, so most oncologists run it while you and your dog wait.
… And Test White Blood Counts After Chemo
Similarly, I recommend checking what is called a nadir CBC after chemotherapy treatment. Nadir is a fancy word that simply means “lowest.”
The chemo agents don’t necessarily destroy white blood cells immediately on contact. It can take a while. For most chemotherapy drugs, it takes about seven days to see just how low the white blood cells get. We call this day the “nadir.”
That’s why I have my patients come in on the 7th day after chemo, to make sure the WBC is not too low.
For drugs that have different nadirs, we schedule that test for a different day. For example, the nadir for carboplatin is 10 to 14 days after treatment. When I give carboplatin, I schedule the nadir CBC during that window, because checking on day seven is inaccurate and a waste of time and money.
And yes, I test on the nadir day even if the dog is acting normal.
Most owners are surprised to find out their dog has a low white blood count (WBC), or neutropenia, at the nadir appointment. That’s because dogs don’t really behave differently when they have a low WBC. Unless they have an infection, they look totally fine.
And they usually are, because white blood cells typically recover really quickly, within a day or two. The bone marrow is continuously making new white blood cells. Low levels don’t harm a dog — as long as there is no infection.
That’s why I also take your dog’s temperature during the nadir appointment, to see if there is an underlying infection we don’t see yet.
Normal Temperature = Good
If the WBC is low, but the temperature is normal, and the dog is feeling well, that’s good news. I just prescribe oral antibiotics to prevent infection and send the dog home.
Elevated Temperature = Worry a Little
If the white blood cell count is low AND your dog has a fever, and especially if he or she is not feeling well, that’s a different story. Then, your dog might need to be admitted for intensive care – typically IV fluids and IV antibiotics.
Editor’s Note: Dr. Sue discusses sepsis (a systemic infection) in Part 3.
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Some chemotherapy drugs are more likely to cause a low white blood cell count at the nadir. For these drugs, I often send a dog home with prophylactic (preventative) antibiotics the first time the drug is given. I routinely do this with Lomustine (CCNU), carboplatin, and sometimes Adriamycin (doxorubicin). Those are the biggies.
Planning Future Doses
Here is another reason to test the WBC at the nadir, even when your dog is acting normally:
If the white blood cell counts are low at the nadir, future chemotherapy doses should probably be adjusted.
We want to give the biggest dose we can that doesn’t lower WBC too much.
And/or, we want to get those prophylactic antibiotics on board right away, with the next treatment.
By testing at the nadir, we know how your pup is responding, and we can adjust the dose and/or make sure antibiotics are used next time.
I know you’re wondering why we don’t prescribe prophylactic antibiotics all the time. The answer is simple: after decades of using antibiotics, we now know that using them unnecessarily is not a good idea. Antibiotic resistance is an issue, and antibiotics can cause GI upset themselves. So I like to use them only when necessary.
What Can Be Done To Correct Low White Blood Cells?
As I mentioned above, the bone marrow keeps churning out white blood cells, and within a few days of chemo treatments, the recovery has already begun. Rarely, we will use WBC stimulators like Neupogen to stimulate the bone marrow.
Remember, the goal of all this is to maximize chemotherapy’s efficacy (kill cancer cells) and minimize the risk of side effects and infection for your dog.
In my next article, I will discuss infections and sepsis that result from neutropenia (chronically low neutrophil counts). I will also discuss why dogs that get low WBC counts may actually do better on chemotherapy.
Until then, be sure to check out more about chemotherapy and its side effects in the Dog Cancer Survival Guide.
All my best,
Dr. Sue Cancer Vet
Sue Ettinger, DVM. Dip. ACVIM (Oncology). Dr. Sue is a boarded veterinary medical cancer specialist. As a Diplomate of the American College of Veterinary Internal Medicine (Oncology), she is one of approximately 400 board-certified veterinary specialists in medical oncology in North America. She is a book author, radio co-host, and an advocate of early cancer detection and raising cancer awareness. Along with Dr. Demian Dressler, Dr. Sue is the co-author of The Dog Cancer Survival Guide: Full Spectrum Treatments to Optimize Your Dog’s Life Quality and Longevity.