Oncology Update: The Nadir CBC

Emergency Veterinarian Toronto

Oncology Update
Central Toronto Veterinary Referral Clinic
Kevin Finora DVM, Diplomate ACVIM (Oncology and Small Animal Internal Medicine)

The Nadir CBC

Cytotoxic drug therapy has many effects on the body. Generally, the consequences of chemotherapy are seen in tissues with rapidly dividing cells. The obvious goal is to kill the rapidly dividing cancer cells with the treatments. However, other cell populations will be impacted. One tissue of special concern is the bone marrow, the site where all blood cell lines are produced. When the bone marrow is impacted there can be a decrease in WBC, RBC and platelet counts.

Careful monitoring of the CBC is necessary for all patients receiving chemotherapy. A CBC is checked prior to each chemotherapy treatment. Attention is given to the WBC and neutrophil counts. RBC and platelet counts are important but there is considerable flexibility in what is considered a level acceptable for treatment administration. Mild to moderate anemia or thrombocytopenia would not, generally, be a reason to withhold therapy. Each chemotherapy drug has a specific time after treatment when the WBC blood count will be at its lowest, called the Nadir, or trough, point. The Nadir tends to occur 7 or 10 days after treatment, though some drugs can have Nadirs as far out as 21 to 28 days. It is important to note that GI signs typically occur 3 to 5 days following treatment and usually have resolved by the time the CBC Nadir approaches. Therefore an animal appearing clinically normal may have low WBC and neutrophil counts. Timing is important for checking the Nadir CBC and there are times when I will have owners return to the family veterinarian so the CBC will be evaluated on the exact appropriate day.

A key factor in chemotherapy protocols is the evaluation of the Nadir CBC at the first Nadir.   Interpretation of the Nadir CBC depends on many factors. The most important issues are how the patient is doing at home and the results of the physical examination. Patients who are clinically ill at the Nadir recheck must be treated regardless of the CBC results. Evaluation of the temperature is important, though it must be remembered that animals with very low WBC counts may be unable to produce the cytokines necessary to induce fever. Therefore it is possible to have a patient with a severely low WBC count and a normal temperature.

Most animals will have WBC counts on the Nadir day ranging from slightly low to slightly elevated. Often, high WBC counts are an “over shoot” response of the bone marrow responding, appropriately, to a declining WBC count. In most cases slightly elevated WBC counts are not of concern. An “over shoot” response does not typically cause a left shift. If a shift is noted, searching for a source of infection is indicated. The key Nadir CBC question is “How low is too low?”

In general, a neutrophil count less than 2500 is considered lower than normal. However, most animals will be clinically normal and have a minimal risk of infection if the neutrophil count is between 1500 and 3000. There is a heightened level of concern when the neutrophils fall below 1500. At a count of 1000, most animals will still be clinically well, but there is an increased risk of infection. These patients should be covered with prophylactic antibiotic therapy. In general I use potentiated amoxicillin but TMS is also an appropriate choice. As long as the animal is not clinically ill they can be managed as an out-patient. In general neutrophil counts will recover back towards normal in 1 to 3 days.

When a neutrophil count drops below 850-1000, the level of concern is high and this situation should be considered an emergency. These animals require hospitalization, reverse isolation, IV fluids and broad-spectrum antibiotics. In some cases granulocyte colony stimulating factor (GCSF) treatment is necessary. GCSF stimulates the bone marrow to produce white blood cells. Typically only one treatment will cause the neutrophil count to begin to rise. GCSF treatments are continued daily until the WBC count starts to increase, at which point treatments can be stopped. GSCF treatments are contraindicated in animals with leukemia. With hospitalization and aggressive therapy, most patients can be released from the hospital in 2 to 3 days. Future chemotherapy treatments are adjusted so as to avoid such low Nadir CBC levels.

If you find yourself evaluating a Nadir CBC and have concerns about the results or feel treatment steps are necessary, always feel free to contact us at the Central Toronto Veterinary Referral Clinic.

Dr. Kevin Finora is a Board Certified Oncologist and Small Animal Internist who is part of the Healthcare Team at the Central Toronto Veterinary Referral Clinic. He is available for referrals and consultations Monday to Thursday (including Monday and Tuesday evenings). Please contact him with any oncology questions or concerns.

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