UNDERSTANDING BLOOD RESULTS - COMPLETE BLOOD COUNT

[Jo CIMDA]

Complete Blood Count
The CBC measures the number of cells of different types circulating in the blood.  There are three basic types, red cells, white cells and platelets.  Red cells are made in the bone marrow (the soft center of the bones).  Their function is to carry oxygen from the lungs to the cells throughout the body.  They are removed from the blood by the spleen and liver.  If a Beardie has too few red cells it is anemic, too many and it has a condition called polycythemia.  Anemia can occur if red cells are lost to either internal or external bleeding; or if they are destroyed earlier than normal (hemolysis). It may also be the result of insufficient production by the bone marrow.  Polycythemia is less common, and usually is the result of dehydration.  Animals living at higher elevations make more red cells to compensate for the lower amounts of oxygen in the air.  As well as the absolute number of red cells, the CBC will include the hematocrit (Hct) or packed cell volume (PCV) – the percentage of red cells in the blood sample.  Blood is spun in very thin tubes, and the red cells settle to the bottom.  Above them is the small "buffy" coat layer of white cells and above that the plasma which should be clear and colorless.  Clear yellow plasma indicates liver disease – jaundice, white opaque plasma indicates lipemia, and pink to red clear plasma the presence of hemoglobin from lysed red cells.  Hemoglobin concentration (Hb) is also measured.  Hemoglobin is the substance in the red cell that carries the oxygen.  Mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC) are all used to help classify anemia, albeit the most useful measure of anemia is the reticulocyte count.  Reticulocytes are immature red cells that still contain a nucleus.  In anemia they may be released prematurely to help meet the animal's need for oxygen.  You need the absolute reticulocyte count not the proportion.  As the numbers of adult red cells drop the proportion of reticulocytes will rise, and so be less informative. 

There are several different kinds of white blood cells or leukocytes.   The CBC will usually report the percentage as well as the absolute number.  The latter is always the more important number.  White cells are also made in the bone marrow. 

Neutrophils
The most numerous white cell group are the neutrophils.  They are phagocytes – cells that can engulf and digest foreign substances, and their primary function is to destroy microbes.  They secrete various substances to help with this, and can also pass through cell walls, attracted to the foreign invaders. Increased numbers of neutrophils indicate inflammation, infection by bacteria, distemper virus, fungi, some parasites, as well as immune mediated disease, necrosis, endotoxins, foreign bodies, hemorrhage, hemolysis and Rocky Mountain Spotted Fever.  Prednisone and other corticosteroid drugs can increase neutrophil numbers, by reducing their stickiness and random migration from blood to tissues.  In general, the response is greatest if the infection is localized (pyometra for example) rather than generalized.  Numbers are low if destruction exceeds production.  This can result from massive utilization but most often is due to decreased production or decreased survival.  Chemotherapy, leukemia, ehrlichiosis, parvovirus, immune mediated diseases, endotoxins or an overwhelming sepsis could cause this. 

Lymphocytes are found in lymph nodes, spleen, thymus, tonsils, lymphoid tissue in the gastrointestinal tract and respiratory system and bone marrow as well as the blood.  While they cannot be differentiated morphologically they are of two types T lymphocytes that mature in the thymus and are involved in cell-mediated immunity and B-lymphocytes that function as antibodies in the blood.  They are the most long lived white cells and are unique in that they recirculate back into the blood from the tissues.  Recirculation is the process most likely to influence the number of lymphocytes found in the blood.  Blood count correlates poorly with enlargement of the lymph nodes. Lymphocytosis (increased numbers) are occasionally seen during chronic infections.  Lymphopenia (low numbers) are relatively common in sick animals, however.  Stress mediated by corticosteroids (natural or drugs) causes lymphocytes to move from the blood into lymphoid tissue.  This effect is maximal 4 to 8 hours after the appearance of the corticosteroids.  Acute infection, immunosuppression, acquired deficiency as well as loss or damage to lymph tissue can all reduce lymphocyte numbers.  In puppies, lymphocytes may be elevated due to infection.

Monocytes usually parallel neutrophils and increase in infection as well as in response to corticosteroids.  Monocytes transform into macrophages after a time in circulation, these are large efficient macrophages with lots more granules and proteolytic enzymes than monocytes.  Macrophages clean out any foreign particles or dead cells, but are less responsive to infection than neutrophils.  They also present foreign substances to lymphocytes in a form more likely to elicit an immune response.  Reduced numbers of monocytes are not clinically significant.

Eosinophils are attracted to substances released by mast cells, and inhibit their further release limiting or delaying allergic or anaphylactic reactions.  Increased level is usually associated with parasitic infection or hypersensitivity.  Reduced numbers can be the result of an acute infection or the presence of corticosteroids. 

Basophils are usually quite rare.   Seeing them is therefore quite significant.  They tend to parallel eosinophils, and tend to increase in dirofilariasis the early stage of heartworm.  If eosinophil levels are normal but basophil levels are high, serious chronic disease should be investigated.

Platelets, also known as thrombocytes, stick to exposed collagen within seconds after injury and help to form clots to prevent internal and external bleeding.  Reduced numbers indicate either bone marrow damage or increased rate of destruction.  The two major causes of destruction are immune-mediated thrombocytopenia (ITP), which may be secondary to tick borne and other infections, or disseminated intravascular coagulation (DIC), a complex and usually terminal condition in which blood clots throughout the body as the result of a number of serious conditions.  Platelet numbers have to drop dramatically before you see spontaneous bruising – including pinpoint petechiation – and bleeding. 

A blood smear will be examined microscopically not only as confirmation of the machine generated cell numbers, but also to look for parasites, as well as abnormal cell shapes and arrangements.  Platelets may clump in samples giving false low readings and the blood smear will determine whether adequate numbers are indeed present.


Linda Aronson, DVM.