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Complete Blood Count

What is a Complete Blood Count?

"... one of the most basic blood tests your veterinarian can request.."

The Complete Blood Count, usually just called a CBC, is one of the most basic blood tests your veterinarian can request, and yet it is one of the most important tests for determining the health status of a pet. It includes a series of measurements that describe the quantity and quality of the cellular elements in the blood.

How is a CBC performed?complete_blood_count-1

A small sample of blood is collected from the pet and placed in a special tube that prevents the blood from clotting. The sample is usually run through an automated analyzer that counts the cells in the blood and takes various measurements. In addition, a drop of blood is spread thinly on a glass slide (called a blood smear), stained with special dyes, and examined under the microscope to evaluate the appearance of individual cells.

What does a CBC measure?

The CBC provides information about the three types of cellular elements found in blood: red blood cells (also called red cells or erythrocytes), white blood cells (also called white cells or leukocytes), and platelets. The information includes details about the number, size, and shape of the various cell types, as well as any abnormalities that may be present.



What are red blood cells and why are they important? 

"Red blood cells are by far the most numerous cells in blood, and give blood its red color."

Red blood cells are by far the most numerous cells in blood, and give blood its red color. They are responsible for carrying oxygen from the lungs to the tissues of the body. They can do this because of a special protein called hemoglobin that is present in each red cell. Hemoglobin has a strong affinity for oxygen, and as blood passes through the lungs, oxygen binds to hemoglobin in the red blood cells. The blood is then pumped through the body, where oxygen separates from the hemoglobin, leaves the red blood cells and enters the tissues. Since red blood cells and hemoglobin are essential to deliver oxygen, deficiencies or abnormalities in the red cells usually result in tissue stress or damage.


What does the CBC tell us about red blood cells?

a)  Numbers:
The most important thing the CBC tells us is how many red cells there are in the blood and how much hemoglobin they are carrying. Three different measurements are taken, namely the Red Blood Cell count (RBC), hematocrit (HCT) and hemoglobin (HGB). A decrease in the number of red cells and/or a reduction in the amount of hemoglobin is called anemia. This is not a specific disease, but it is a sign of an underlying problem that needs to be investigated. An increase in the number of red cells is usually a sign of dehydration or excitement, although in very rare cases it can signal serious disease.

b)  Size:complete_blood_count-3
The size of the red blood cells sometimes provides clues about a disease. For example, in an anemic animal the presence of larger than normal red blood cells indicates there are young red cells in the blood - a sign that the bone marrow has increased production of red blood cells and is "responding" to the anemia. The veterinarian recognizes that responsive anemia is usually associated with either red cell loss (bleeding) or red cell destruction, and can then follow specific steps to make the diagnosis. In other cases, the presence of smaller than usual red blood cells indicates diseases such as iron deficiency, liver problems, and red cell injury. Sometimes the red cells are the correct size, but there just aren't enough of them. This suggests the presence of a chronic disease, such as chronic renal disease, chronic inflammation, or cancer.

c)  Appearance:
The color and shape of the erythrocytes are additional details that help in the diagnosis of disease. For example, an erythrocyte with a bluish tinge is called a polychromatophilic erythrocyte (often shortened to polychrome); it is a young red cell newly released from the bone marrow. The number of polychromes in the blood is an important indicator of how actively the bone marrow is producing red cells. Sometimes a special dye is used to identify young red cells, and the number is reported as areticulocyte count. It provides the same basic information as counting polychromes, but it is more accurate.

Many other red cell shape changes have been associated with various diseases. Although they are rarely diagnostic by themselves, red cell shape changes can often provide clues to the nature of the underlying disease.

What are white blood cells and why are they important?

White blood cells or leukocytes ("leuko" means 'white') are essential to help protect the body against infectious organisms, such as bacteria and viruses, or against foreign material that may enter the body through various routes.

"White blood cells like the "armed forces" of the body..."

It sometimes helps to think of white blood cells like the "armed forces" of the body, with different branches that have different roles. Just like a country's armed forces have an army, a navy, and an air force, the armed forces of body have five types of white blood cells that are called neutrophils, lymphocytes, monocytes, eosinophils, and basophils. These different groups of leukocytes respond independently or in combination, depending on the disease or problem that is present.


What does the CBC tell us about white blood cells?

a)  Numbers:complete_blood_count-4
As with the red blood cells, the automated analyzer counts the total number of white blood cells and reports a White Blood Cell count (WBC), which gives an indication of the pet's general level of protection against infection. The total number of white blood cells is then subdivided to give the number of leukocytes in each of the five divisions. These five numbers together are called thedifferential. Changes, either up or down, in the numbers of any component in the differential provide information about the type and severity of inflammation, possible causes of the problem, and the bone marrow's ability to meet the demand for leukocytes. Sometimes there are marked elevations or reductions in the number of white blood cells, which can be an indication of bone marrow cancer or leukemia.

b)  Appearance:
Studying the appearance of the leukocytes tells us if the body is responding well to inflammation or is struggling, either because of the severity of the problem or the presence of toxins. It can also tell us if the immune system has been activated, or if there are cancer cells in the bloodstream.



What are platelets and why are they important? 

Platelets are small cell-like structures that are present in large numbers in blood. Their main role is to help with blood clotting, especially with small wounds. For example, when you get a scratch or paper cut, or prick your finger with a pin, it is the platelets that stop the bleeding.

"Platelets are the first line of defense against bleeding..."

Platelets are the first line of defense against bleeding, and they are continually on guard to seal microscopic injuries that occur within blood vessels. It is vital to maintain adequate numbers of platelets in the blood.


What does the CBC tell us about platelets?

a)  Numbers:
The analyzer counts the number of platelets in the blood, and the reported platelet count gives a general indication of the clotting ability of the blood. If the number of platelets falls below a certain critical level, spontaneous bleeding may occur. A low platelet count may indicate a problem with platelet production in the bone marrow, or may signal the presence of disease that is causing the platelets to be used up or destroyed. An increased platelet count often reflects excitement, exertion, or an activated bone marrow. In rare cases, an extremely high platelet count may indicate there is underlying bone marrow cancer.

b)  Size:
The size of a platelet is related to its age; young platelets are large and plump, and older platelets are generally smaller. By looking at the size of the platelets in a blood sample, a veterinarian can assess how actively the bone marrow is producing new platelets. This can be important if the platelet count is low; the presence of large, plump, young platelets in the blood indicates that the bone marrow is functioning well, and is responding to the need for more platelets. This helps the veterinarian decide what further testing is needed to find the cause of the low platelet count.

c)  Appearance:
Very rarely, bizarre giant platelets, or abnormal immature platelets may be found, and these may signal the presence of an underlying bone marrow disorder or cancer.

This client information sheet is based on material written by: Kristiina Ruotsalo, DVM, DVSc, Dip ACVP & Margo S. Tant BSc, DVM, DVSc

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