The complete blood cell count (CBC) and its differential provide a wealth of information to the veterinary practitioner. As an oncologist, it is of utmost importance in monitoring a patient’s tolerance of a chemotherapy drug and his risk for complications associated with bone marrow toxicity. The primary concern is typically for sepsis as a consequence of neutropenia. However, anemia (a low red blood cell count) and thrombocytopenia (a low platelet count) can also occur secondary to chemotherapy. CBCs can reveal infiltration of bone marrow by cancers such as mast cell cancer or leukemia. CBCs can aid in the diagnosis of paraneoplastic syndromes (syndromes triggered by cancer but not cancer themselves) such as immune mediated thrombocytopenia or anemia.

A CBC is useless without a differential.

Knowing the total white blood cell count (WBC) without knowing the breakdown of types of white blood cells comprising that total WBC count could lead you completely astray. The total WBC count of a CBC is a summation of the number of neutrophils, lymphocytes, eosinophils, monocytes, and basophils. The absolute values of these cell counts is far more important than the relative percentage of these cell types, except in rare circumstances. As you can imagine, a CBC with a WBC count of 53,000 consisting of a neutrophil count of 50,000 and lymphocyte count of 3,000 is very different than a neutrophil count of 3,000 and a lymphocyte count of 50,000.

Neutropenia (low neutrophil count):
The neutrophil is one of the most important and primary type of white blood cell. When treating a patient with chemotherapy, it is important to know the baseline neutrophil count. Neutrophils are the first line of defense against bacterial infection and one of the primary cells damaged by chemotherapy. The suppression of this part of the immune system by chemotherapy increases the patient risk for infection or sepsis. When we talk about immune system suppression by chemotherapy, this is primarily what we are worried about. In a small percentage of patients and depending on the drug used, chemotherapy can cause neutrophil counts to drop below 500/ microL. Patients are at increased risk for bacterial infection when their neutrophil count drops below 1000. The typical time point for this is 7 to 10 days after chemotherapy. This time point is called the nadir, or when the neutrophil count is the lowest following treatment. However, neutropenia can occur as early as 4 to 5 days following Vinblastine and as late as 2 to 3 weeks after carboplatin. The bacteria patients are most at risk for infection from are the natural flora on their own skin, lungs or intestinal tract. However, it is still prudent to reduce their risk of contact with external infectious agents (raw diets, stagnant water, other animals with known infectious diseases).

Should a patient’s neutrophil count drop below 1200, further chemotherapy should be discontinued or delayed. If the count is 1000 or less, they should be started on broad spectrum antibiotics, even if they are not febrile. Some patients with neutropenia are often afebrile and asymptomatic, even with a neutrophil count less than 1000/ microL. These patients should be treated as outpatients and started on broad spectrum antibiotics such as TMS or a fluoroquinolone. The client should be instructed to monitor their pet’s temperature every 12 to 24 hours or if the pet starts to act ill. If the neutrophil count is over 1000, no therapy is needed. With neutropenic patients that are clinically ill, inappetent, depressed and/ or febrile, the patient should be hospitalized for fluids and antibiotics.

We routinely advise our patients to have a CBC done the week after a treatment is given, around the expected time of the nadir, to monitor for neutropenia. Often clients prefer to go to their primary care doctor to have this test done. It is important that the CBC is run in house, so that we get the information back immediately, so that if a patient is neutropenic and requires antibiotic intervention, we can start antibiotics immediately, rather than having to wait for a lab report that might sit on a desk all weekend, delaying intervention in a potentially life threatening situation.

The prophylactic use of antibiotics prior to the onset of neutropenia has not been shown to reduce the risk of neutropenia or aid in recovery.

Metronomic chemotherapy and tyrosine kinase inhibitor therapy can also lead to neutropenias or thrombocytopenias both with short term and long term usage. These patients should be monitored closely for downward trends in blood counts, so that treatment may be discontinued before a life-threatening complication occurs. CCNU (Lomustine) is an oral chemotherapy drug that is commonly seen to cause bone marrow toxicity and low blood cell counts. It should never be prescribed without regular monitoring of blood work.

Anemia: low red blood cell count

It is not uncommon for patients on chemotherapy to have a decrease in their red blood cell count. Red blood cells are responsible for carrying oxygen around the body. A patient who is mildly anemic may tire more easily. Unless the anemia is severe, no action is typically taken. If a patient has a severe anemia, I would be concerned about blood loss (GI bleeding, tumor bleeding, immune mediated disease, etc) or kidney disease, rather than chemotherapy as the cause.

Thrombocytopenia: low platelet count

Platelets are the cells that make blood clots. This is an ongoing activity in the body that prevents blood vessels from becoming leaky. Without enough platelets, bruising and or bleeding may occur. Severe thrombocytopenia can cause life threatening internal or external bleeding. It is important to monitor for chemotherapy induced cytopenias and discontinue chemotherapy before the platelet counts drop too low. Common chemotherapy drugs that may cause significant thrombocytopenia include carboplatin, CCNU, and chlorambucil, among others.

A CBC as a reflection of bone marrow disease

If a patient has cancer in her bone marrow, a CBC can be helpful in monitoring response to therapy. Otherwise, the CBC does not tell us much about remission, as much as inform us to how well a patient is tolerating the therapy. The most common bone marrow cancers seen are lymphoid leukemias, both acute and chronic in nature. Clinical pathology evaluation of cells can be helpful in distinguishing between types of leukemia. Flow cytometry may also be beneficial in understanding the type of leukemia present. In rare cases, patients with mast cell cancer will have increased mast cells in circulation or concurrent hypereosinophilia or neutrophilia. It is important to look at the whole CBC in patients with leukemia. As bone marrow cancer starts to obliterate healthy progenitor cells in the bone marrow, the bone marrow loses the ability to produce normal amounts of neutrophils, platelets, and red blood cells. As the bone marrow health declines, patients are at risk for infection, bleeding, and organ failure. Knowing what the other cell counts are doing in the face of leukemia helps to assess patient risk for these complications.

A CBC to diagnose paraneoplastic syndromes

The two most common paraneoplastic syndromes that can be diagnosed on a CBC are immune mediated thrombocytopenia (IMTP) and immune mediated anemia (IMHA). These findings are not specific for cancer, but can often suggest that there is some underlying disease process that needs to be identified in order to help manage these problems. If the underlying problem is found and treated, then IMTP and IMHA often resolve concurrently. IMTP and IMHA can alone be life threatening problems, but can be treatable if the underlying problem is treated.

The CBC is an important monitoring tool to make sure patients are being treated appropriately and safely. Complete obliteration of bone marrow progenitor cells by chemotherapy or bone marrow cancer can result in bone marrow failure and death. Appropriate dosing and close monitoring of patients undergoing chemotherapy treatments is important so that treatment may be discontinued before irreversible damage is done. The best treatment for bone marrow recovery is time. If you have more questions, do not hesitate to call your veterinarian.