Why does my pet need a complete blood count?

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Why does my pet need a complete blood count?

The complete blood count (CBC) is a common blood test that is performed during a routine wellness check or as a tool to diagnose a medical issue. A CBC can be easily performed at most veterinary facilities. Continue reading to learn about the components of a complete blood count and how your vet uses this tool to evaluate your pet’s health.

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What is a complete blood count?

During your pet’s physical exam, routine blood work is often performed to provide more comprehensive information about your pet’s health. The test is easy to perform and can give a lot of vital information about the different types of blood cells - red blood cells (erythrocytes), white blood cells (erythrocytes), and platelets (thrombocytes) - in the body. The CBC can also be used as one of the tools to evaluate bone marrow health because this is where blood cells are formed.

The results of a CBC provide valuable information about the number, size, and shape of the three main types of cells in the blood. Any variation in the appearance of the cells can also be detected by this blood test.

Three Types of Blood Cells

1. Red Blood Cells (RBCs)

Also called erythrocytes, RBCs are the most abundant cells in the blood, composing approximately 40-45%. These cells have no nucleus and can change shape without breaking. Hemoglobin comprises 33% of an RBC. The iron that hemoglobin contains is responsible for the red color of blood. Hemoglobin is important for carrying oxygen in the blood from the lungs to the tissues of the body and pick up carbon dioxide which is the waste product of tissues so it can be eliminated via the lungs.

2. White Blood Cells (WBCs)

Also called leukocytes, WBCs account for only about 1% of the blood. These cells primarily make up the immune system. White blood cells protect and combat foreign substances and infections. Like RBCs, these cells are produced in the bone marrow.

A low white blood cell count is called ‘leukopenia’ and can indicate damage in the bone marrow caused by chemotherapy, medications, or radiation.

A high white blood cell count is termed ‘leukocytosis’. The abnormality can be caused by several different conditions including infections or inflammatory processes in the body.

There are two types of white blood cells-- granulocytes and agranulocytes.


  • Neutrophils - Also called polymorphonuclear cells, neutrophils are the most common types of white blood cells. They can kill bacteria through a process called phagocytosis.
  • Eosinophils - Functions to kill parasites and play an important role in allergic reactions. Eosinophils also release toxins to kill pathogens.
  • Basophils - Has an important function in allergic reactions. Basophils secrete antibodies and anticoagulants that protect the body against hypersensitive reactions in the blood circulation. The histamine in basophils causes the dilation of blood vessels to bring more immune cells to a part of the body where an injury is present. Basophils also secrete heparin which is an anticoagulant and prevents clotting.


  • Lymphocytes - There are 2 types of lymphocytes: The T lymphocytes (T-cells) and the B lymphocytes (B-cells). Both of them are responsible for mounting an immune response. They can recognize and retain a memory of invading pathogens in the body. Lymphocytes can also help destroy cancer cells.
  • Monocytes - These are the largest types of white blood cells. Monocytes can exit the bloodstream and enter tissues where they become macrophages. They also function to destroy old, damaged, and dead body cells.

3. Platelets (Thrombocytes)

Platelets play an important role in the blood clotting mechanism of the body. They also secrete certain chemicals that attract neutrophils and monocytes to areas of the body where inflammation is present. Platelets dissolve blood clots when the body no longer needs them. They can also digest and destroy bacteria. Growth factors secreted by platelets help maintain the integrity and function of blood vessel linings.

Why did the vet recommend blood work for my dog?

The results of a comprehensive physical exam coupled with a CBC can help your vet determine the best treatment plan for your pet. The CBC results can also help determine if there is a need for further tests and/or procedures to arrive at a diagnosis.

With a CBC, early detection of illness in your pet is possible even if you haven’t noticed any symptoms. With early detection, your pet’s health issue can be given appropriate treatment and attention. Also, during your pet’s wellness visits, the result of the CBC can provide information about your pet’s normal baseline values that your vet could use for comparison later.

If your pet needs to undergo a certain procedure, blood work is generally recommended. Pre-anesthetic bloodwork typically involves a CBC and a chemistry panel. The results of these tests provide valuable information about your pet’s health status. The CBC results can also help determine if your pet has certain surgical or pre-anesthetic risks.

What Does A CBC Measure?

A CBC evaluates the quantity and quality of red blood cells, white blood cells, and platelets in the blood. Any abnormality in the cell count, size, and/or shape may indicate an underlying health issue. It can reveal if your pet is dehydrated, anemic, or if there is an underlying infection. A CBC test result typically shows the following information:

Hematocrit (HCT) - Indicates the number of red blood cells that are present. Anemia is often suspected if the HCT is low, while a high HCT could mean your pet is dehydrated.

Hemoglobin (Hbg) - Measures the amount of hemoglobin; this can help assess the oxygen-carrying capacity of the red blood cells to the body’s tissues.

White Blood Cell Count (WBC) - Indicates the total number of white blood cells. The presence of an inflammation or infection in the body can cause the number of WBCs to increase. On the other hand, a low WBC count could indicate several things including serious infection, or a problem in the bone marrow that is limiting the white blood cell production. There are several types of white blood cells, and each responds distinctly to different events in the body. For example, when a dog or cat is suffering from allergies or has a heavy parasite infestation, the eosinophils (EOS) are usually elevated.

Platelet (PLT) - Indicates the number of platelets (platelet count). Platelets are important factors in the blood clotting mechanism of the body. A patient with a low platelet number may have developed problems that prevent the body’s ability to form blood clots.

How is a CBC performed?

A CBC is performed by collecting a small sample of blood into a special tube that contains an anti-clotting agent. The blood sample is then placed in an automated blood analyzer, a machine that counts and describes the various characteristics of different cell types.

It is also customary to make a blood smear by getting a drop of blood from the sample and spreading it thinly on a glass slide. Special dyes are used to stain the blood smear and make it possible to examine the appearance of blood cells under a microscope.

Some CBCs take only a few minutes. With immediate results, treatment can be started right away. If the results are abnormal, there may be a need to run more tests while you’re still in the clinic. This can mean fewer trips back and forth to the clinic. If the CBC results are normal, certain illnesses can be ruled out by your vet right away!

Read more:

Why does my pet need an echocardiogram?

What You Need to Know About Your Pet’s Liver

Why does my pet need a urinalysis?

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Published: 9/7/2021

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