Complete Blood Count

Complete blood count (CBC)
The complete blood count measures the number of cells of different types circulating in the bloodstream. There are three major types of blood cells in circulation; red blood cells (RBC), white blood cells (WBC), and platelets. Red blood cells are produced in the bone marrow, which is the soft center of bones. RBCs pick up oxygen brought into the body by the lungs, and bring that oxygen to cells throughout the body. Red blood cells live in the blood stream for about 100 days although the actual time varies with the type of animal. Old red blood cells are removed from the blood stream by the spleen and liver. Red blood cell numbers can be decreased (anemia) if they are not produced in adequate numbers by the bone marrow, if their life span is shortened (a condition called hemolysis), or if they are lost due to bleeding. Increased red blood cell numbers is called polycythemia and is usually due to concentration of the blood due to dehydration. 

The complete blood count also includes a measure of hemoglobin, which is the actual substance in the red blood cell that carries oxygen.

There are several types of white blood cells in blood, including neutrophils (PMNs), lymphocytes, monocytes, eosinophils and basophils. Lymphocytes are produced in lymph nodes throughout the body. The other white blood cell types are produced in the bone marrow along with the red blood cells and platelets. The majority of white blood cells in circulation are neutrophils, which help the animal fight infections. Neutrophils can be decreased in pets with bone marrow disease, in some viral diseases, and in some pets receiving cancer chemotherapy drugs. Neutrophils are increased in pets with inflammation or infection of any part of the body and in pets receiving prednisone or other cortisone-type drugs. 

Lymphocytes also help fight infection and produce antibodies against infectious agents (viruses, bacteria, etc.). Lymphocytes may be increased in puppies and kittens with an infection, they can be decreased in pets who are severely stressed, and lymphocytes might be lost in some types of diarrhea. Certain drugs, such as prednisone (a cortisone-type drug) will decrease the number of lymphocytes in the blood stream. 

Monocytes may be increased in pets with chronic infections. Eosinophils and basophils are increased in pets with allergic diseases, or parasitic infections (worms, fleas, etc.). 

Platelets are produced in the bone marrow and are involved in the process of making a blood clot. Platelets live a few weeks and are constantly being produced by the bone marrow. Low platelet counts occur if the bone marrow is damaged and doesn’t produce them, or if the platelets are destroyed at a faster rate than normal. The two primary causes of platelet destruction are immune-mediated destruction (ITP or IMT) and DIC (disseminated intravascular coagulation). Immune-mediated thrombocytopenia happens when the animal’s immune system destroys platelets. DIC is a complex problem in which blood clots form in the body using the platelets faster than the bone marrow can produce new ones. Animals with a low platelet count bruise easily and may have blood in their urine or stool.

Packed cell volume (PCV) (called hematocrit, HCT, in humans) is another measure of red blood cells. A small amount of blood is placed in a tiny glass tube and spun in a centrifuge. The blood cells pack to the bottom of the tube and the fluid floats on top. The PCV is the percent of blood, that is cells, compared to the total volume of blood. In normal dogs and cats, 40-50% of the blood is made up of blood cells and the remainder is fluid.