Abstract

Complete blood count (CBC) is conducted as an essential part of screening work-up for both healthy and sick patients. The aim of CBC is to detect systemic abnormalities and selected significant disease processes. In order to achieve these goals, the CBC must be complete rather than a simplified form as in the emergency situations, and ideally should be performed in the hospital for immediate evaluation. The CBC consists of data generated from an automated blood cell counter, microhematocrit, blood smear evaluation and some determinations using plasma.

Why is CBC Necessary?

The screening laboratory examinations usually consist of CBC, blood chemistry panel and urinalysis. They are performed at the same time and will tell the clinician the type and the location of disease processes. The screening tests are usually planned to evaluate the presence of systemic or hidden illnesses. While blood chemistry screening suggests the location of the illness, the CBC tells you the type of the disease. By evaluating the white cell parameters, it is possible to make an assessment whether there are evidences of inflammation, necrosis, stress or hypersensitivity. Also in some instances, it may be possible to detect evidences of bacterial infection or blood-related malignant diseases. In the red cell evaluation, it is possible to detect anemia and erythrocytosis, and in most situations, the etiology can be detected. Evaluation of the plasma component in the microhematocrit will provide the clinician with information regarding to the plasma proteins and icterus. The blood smear evaluation will provide morphological clues to the certain diagnoses.

What Parameters Should be Evaluated?

In order to achieve the complete evaluation, a complete set of parameters is necessary. Table 1 shows the parameters to be obtained in CBC. The three basic red cell parameters, RBC (red blood cell count), PCV (packed cell volume), Hb (hemoglobin concentration), are necessary to see if there are evidences of anemia or erythrocytosis. The reason why we evaluate all three parameters is that we can not rely on a single parameter. The red cell indices, MCV (mean cell volume), MCH (mean cell hemoglobin amount) and MCHC (mean cell hemoglobin concentration), have been used for morphological classification of anemia, but blood smear evaluation is definitely more reliable. Reticulocyte count (Ret) is necessary to classify the anemia into regenerative and non-regenerative. It is important to remember that the Ret is not a percentage but should be evaluated as an absolute count.

White cell parameters include absolute numbers of total white cells (WBC) and those of each cell type. The differential white cell count should be expressed in the absolute numbers but not percentage. The platelets are also counted as absolute numbers. Some instruments report mean platelet volume (MPV) and the total mass of platelets (Platelet crit), and these are helpful when platelets vary in size.

The reason why we need microhematocrit tube evaluation in CBC even though the instrument would generate all the necessary information regarding the blood cells is that it serves as a great monitoring device for the hematology instrument. It provides the red cell data as a hematocrit, and the white cell data as a buffy coat. Also, the CBC should include some evaluations of plasma components such as total plasma protein concentration (TP) and icterus index (II). The TP is especially important to follow PCV values, because it provides valuable information regarding dehydration or blood loss.

How is CBC Performed?

Blood is obtained via venipuncture from the cervical vein into a disposable plastic syringe with a 22 or 23 G needle. It is important to remember that blood is collected in a syringe with no anticoagulant. Then the blood is placed in a tube containing potassium EDTA, and the rest can be placed in a tube containing lithium heparin or a serum tube for chemistry.

First, blood smear is prepared by either a glass slide or a coverslip smear method. Then, methanol fixation and Wright-Giemsa staining is started right away. Two microhematocrit tubes are prepared and centrifuged. The blood is then introduced into either an impedance-based automated blood counter or a laser flow cytometry-based blood counter. The laser-based counters (LaserCyte or ProCyte DX) will provide five-parts differential white cell counts as well as reticulocyte counts.

In the mean time, the hematocrit is read with a scale, II is determined by using a color scale, and the plasma protein concentration is determined by using a refractometer.

After all the data are obtained, the stained blood smear is examined for morphological evaluations of the blood cells. A differential count is performed by counting at least 200 white cells under a high power (400X) view. In case of laser flow cytometry, only a brief examination for morphology or for an evidence of left shift is usually necessary, unless the instrument reports any abnormalities that need confirmations on the smear.

How Are the CBC Data Evaluated?

The CBC data are evaluated in a systematic manner. As stated above, there are certain abnormalities we would like to detect in the CBC.

Is There An Evidence of Inflammation?

If band neutrophils exceed 300/µL, or exceed 1,000/µL with 40,000 or more segmented neutrophils, monocytes exceed 2,500/µL, or eosinophils exceed 1,500/µL, it is considered that there is an inflammation.

What Type of Inflammation?

The presence of an increased number of band neutrophils (left shift) indicates acute inflammation. In these cases, segmented neutrophils are usually not increased or even decreased. There are two types of left shift patterns; non-regenerative left shift usually indicates overwhelming infection, and is characterized by a left shift with normal to decreased numbers of segmented neutrophils. Presence of neutrophil toxic changes (Dole bodies, basophilic or vacuolated cytoplasm) usually supports bacterial infection. Regenerative left shift is characterized by an increased number of segmented neutrophils, and indicates established or chronic active inflammation. Increased numbers of segmented neutrophils and monocytes indicates chronic inflammation. Increased number of eosinophils usually suggests eosinophilic inflammation such as hypersensitivity or parasite infestation, but one needs to remember that malignancies such as mast cell tumor, lymphoma, or body cavity dissemination of carcinoma occasionally produce eosinophilia.

Is There An Evidence of Necrosis?

An increase in the number of monocytes may indicate presence of necrosis. This is especially true if neutrophil count suggests acute inflammation but not chronic inflammation.

Is There An Evidence of Stress/Pain?

Decreased numbers of lymphocytes (less than 1,500/µL indicates presence of stress or pain. This is usually associated with a decreased number of eosinophils, but eosinopenia is not a sensitive indicator of stress if previous eosinophil count is unknown. If there is a severe decrease in lymphocyte count (less than 400/µL), it is possible to consider that there is something else such as immunosuppression or viral infection (FeLV, FeLV).

Other White Cell Changes?

Lymphocytosis, or increased numbers of reactive lymphocytes, may be associated with immune reaction such as postvaccination, recovery from infection, or immune mediated disorders. Increased numbers of granular lymphocytes may indicate viral infection. If the lymphocyte count is greater than 10,000/µL, stage V of low grade lymphoma or chronic lymphocytic leukemia (very rare in cats) can be considered. Presence of abnormal blast cells strongly suggests malignancy such as stage V high grade lymphoma, acute lymphoblastic leukemia or other myeloproliferative disorders. Increased number of basophils is a nonspecific finding in cats, but generally suggests some disease process.

Is There An Evidence of Anemia?

Subnormal values in red cell parameters always indicate anemia. When anemia is present, it is important to classify the anemia into regenerative and non-regenerative. Detection of anisocytosis and polychromasia indicates increased numbers of reticulocytes, and reticulocytosis greater than 100,000/µL indicates regenerative anemia. Regenerative anemia is then subdivided into acute blood loss and hemolysis, and there are many blood smear findings to support hemolysis. These include Heinz body, hemoplasmosis, and acanthocytosis (hepatic lipidosis). Spherocytes, indicating immune mediated hemolytic anemia (IMHA), unfortunately, are not easily detected in the feline blood smear. Non-regenerative anemias usually provide no clues in the blood smear, but iron deficiency anemia shows a characteristic shape of red cells with prominent central pallor. Non-regenerative anemias include anemia of chronic disease, anemia of chronic renal failure, and severe progressive anemias due to bone marrow disorders. Some types of non-regenerative IMHA, however, usually have erythrocytic hyperplasia in the bone marrow, whereas pure red cell aplasia (PRCA), another possible immunologic disease directed against the stem cells of the red cell series, has red cell aplasia in the bone marrow.

Is There An Evidence of Erythrocytosis?

If red cell parameters exceed the higher limit of the reference intervals, one needs to explore the etiology of the erythrocytosis. First, relative erythrocytosis due to dehydration should be ruled out. Next, possibilities for chronic oxygen deficiency and erythropoietin (EPO) production should be ruled out through the respiratory and cardiac evaluations. Then, the possibility of a renal tumor associated with an increased production of EPO should be evaluated. If everything is ruled out, one needs to consider a benign chronic myeloproliferative disorder, polycythemia vera, and bone marrow examination is carried out. The term polycythemia is not necessarily appropriate for the cats since this condition does not usually accompany increases in other cell types.

Is There An Evidence of Thrombocytopenia?

Thrombocytopenia can be associated with disseminated intravascular coagulation (DIC), immune-mediated or non-immune mediated destruction of platelets, increased consumption, abnormal distribution, or decreased production in the bone marrow. It is usually necessary to examine the bone marrow.

Table 1. The Feline CBC Parameters and Reference Intervals.