Complete blood count (CBC), also known as a hemogram, is one of the most frequently used basic laboratory tests in veterinary medicine. The hemogram plays an important role in the diagnosis and monitoring of various diseases by evaluating the quantity and characteristics of blood cells. For hemogram analysis, a purple-top (EDTA) tube is used. This is because the tube contains ethylenediaminetetraacetic acid (EDTA), which prevents blood coagulation by binding calcium. Thus, the blood remains in a liquid state and becomes suitable for analysis.

There are several important points to consider during sample collection, including:

• The tube must be adequately filled; otherwise, an incorrect EDTA-to-blood ratio may lead to erroneous hematological results. The recommended ratio is 1 mg EDTA per 1 mL of blood.
• After blood collection, the tube should be gently inverted several times to mix the contents (it should not be shaken, as this may cause hemolysis).
• Samples may be stored for a short period at room temperature or at 4°C but should not be kept for prolonged periods.

The hemogram evaluates three main cell groups:

1. Erythrocytes (Red Blood Cells)
2. Leukocytes (White Blood Cells)
3. Platelets (Thrombocytes)

In addition, several specific parameters related to erythrocytes and leukocytes are also assessed.

ERYTHROCYTE (RBC – RED BLOOD CELL) PARAMETERS

1. RBC

RBC represents the total number of red blood cells in the blood and is usually expressed as million cells per microliter (M/µL).

• Increased RBC: May indicate polycythemia, dehydration, or a compensatory response to hypoxia.
• Decreased RBC: Associated with anemia, blood loss, bone marrow failure, or chronic diseases.

2. HGB (Hemoglobin Concentration)

Hemoglobin is an iron-containing protein within erythrocytes that is responsible for oxygen transport. It is typically measured in grams per deciliter (g/dL).

• Increased HGB: Seen in dehydration or polycythemia.
• Decreased HGB: Observed in anemia, blood loss, or chronic diseases.

3. HCT (Hematocrit or PCV)

Hematocrit represents the percentage of blood volume occupied by erythrocytes.

• Increased HCT: Seen in dehydration, polycythemia, or hypoxia.
• Decreased HCT: Occurs in anemia or blood loss.

4. MCV (Mean Corpuscular Volume)

MCV measures the average size of erythrocytes and is expressed in femtoliters (fL). MCV and MCHC are clinically valuable indices in the differential diagnosis of anemia.

• Normal (Normocytic)
• Decreased (Microcytic)
• Increased (Macrocytic)
• Increased MCV (Macrocytosis): Regenerative anemia (following blood loss or hemolysis), vitamin B12 or folic acid deficiency.
• Decreased MCV (Microcytosis): Iron deficiency anemia, anemia of chronic disease, or certain breed-related factors (e.g., physiologically low in Akita dogs).

5. MCH (Mean Corpuscular Hemoglobin)

MCH represents the average amount of hemoglobin per erythrocyte and is expressed in picograms (pg).

• Increased MCH: Seen in macrocytic anemias.
• Decreased MCH: Observed in iron deficiency anemia and microcytic anemias.

6. MCHC (Mean Corpuscular Hemoglobin Concentration)

MCHC indicates the average hemoglobin concentration within erythrocytes and is expressed in g/dL.

• Increased MCHC (Hyperchromia): A true physiological increase is not possible; elevated values are always considered artifacts, usually due to measurement errors, though they may also be observed in some hemolytic anemias.
• Decreased MCHC (Hypochromia): Seen in iron deficiency anemia and anemia of chronic disease.

7. RDW (Red Cell Distribution Width)

RDW reflects the variation in erythrocyte size and is expressed as a percentage.

• Increased RDW: Indicates anisocytosis and is commonly seen in regenerative anemias; it may suggest iron deficiency anemia, vitamin B12 or folic acid deficiency, liver disease, or certain hematologic disorders.
• Decreased RDW: Generally not clinically significant.

WHITE BLOOD CELLS (LEUKOCYTES)

Leukocytes are essential components of the immune system and protect the body against infections, foreign substances, and diseases. In a hemogram, both the total leukocyte count (WBC) and the percentages of different leukocyte types are determined.

WBC (White Blood Cell Count)

Represents the total number of white blood cells.

Leukocytosis (Increased WBC)

Defined as an increase in leukocyte count above reference values.

• Physiological leukocytosis: Occurs due to parturition, tachycardia, unfamiliar environments, fear, or increased physical activity.
• Pathological leukocytosis: Associated with local or systemic infections caused by bacteria, fungi, protozoa, or rickettsiae.
  
  • Endogenous intoxication due to intestinal toxin absorption
  • Diabetic ketoacidosis
  • Uremic syndromes
  • Exogenous intoxications (lead, thallium, sodium chloride)
  • Irreversible leukocytosis occurs in leukemic leukosis
  • Observed during recovery from CNS inflammation, tumors, trauma, and shock

Leukopenia (Decreased WBC)

A reduction in total leukocyte count below reference values.

• Viral diseases such as canine parvovirus (CPV) and classical swine fever
• Peracute peritonitis, pleuritis, endotoxemic or anaphylactic shock
• Cytotoxic agents and chemotherapeutics (e.g., vincristine, cyclophosphamide)
• Diseases causing bone marrow destruction

LEUKOCYTE CLASSIFICATION

1. GRANULOCYTES

Leukocytes containing cytoplasmic granules.

• NEU (Neutrophils)
• EOS (Eosinophils)
• BASO (Basophils)

A) Neutrophils

Include band (immature) and segmented (mature) forms. They migrate into tissues through diapedesis and perform phagocytosis via lysosomal enzymes. Due to chemotaxis, they accumulate at sites of tissue injury.

• Physiological neutrophilia: Sympathetic stimulation, intense physical activity
• Pathological neutrophilia: Bacterial infections, endogenous/exogenous intoxications, absorption of tissue or foreign proteins, hypersensitivity reactions, severe hemorrhage

Neutropenia:

• Reduced neutrophil count
• Occurs due to decreased granulocyte production in viral infections
• Associated with lymphocyte proliferation (e.g., septic shock)
• Seen in bone marrow destruction, myeloid leukemia, and bone marrow tumors

B) Eosinophils

• Involved in hypersensitivity reactions
• Rapidly mobilized from bone marrow in autoimmune and allergic inflammatory conditions, causing eosinophilia
• Eosinopenia: Associated with corticosteroid or ACTH secretion, stress conditions, or Cushing’s syndrome

C) Basophils

• Increased in horses with allergic diseases and hyperlipemia
• Increased in dogs with dirofilariasis
• Increased in heavy metal intoxications

2. AGRANULOCYTES

Leukocytes lacking prominent granules.

• LYM (Lymphocytes)
• MONO (Monocytes)

A) Lymphocytes

Play a key role in antiviral defense and antibody production.

• Lymphocytosis: Physiological in young animals; pathologically seen during recovery from infections, chronic inflammation, and viral diseases
• Markedly increased in lymphatic leukosis
• Increased in lymphosarcoma and lymphatic leukemia affecting the hematopoietic system

Lymphocytopenia:

• Seen in chronic inflammation of lymphoid tissues (e.g., distemper)
• Corticosteroid therapy
• Acute stress
• Cushing’s syndrome

B) Monocytes

Monocytes play an important role in evaluating immune status and combating infections by regulating immune responses.

Monocytosis:

• Infections (bacterial, viral such as canine parvovirus, parasitic diseases such as leishmaniasis)
• Inflammatory diseases (connective tissue disorders, arthritis
• Immune-mediated diseases (e.g., lupus)
• Neoplasia (leukemia, lymphoma)
• Severe stress or trauma
  

Monocytopenia:

• Rarely observed
• Bone marrow disorders (chemotherapy, radiation, bone marrow diseases)
• Viral infections
• Severe infections causing immune exhaustion
• Corticosteroid therapy (e.g., prednisone)

PLATELETS (THROMBOCYTES)

Platelets are anucleate, disk-shaped cell fragments produced in the bone marrow and play a crucial role in blood coagulation. Their surface contains receptors and glycoproteins that activate coagulation factors, while platelet granules facilitate clot formation at bleeding sites.

Platelet Parameters

1. Platelet Count (PLT)

Represents the total number of platelets, typically expressed as thousand/µL (K/µL).

Thrombocytosis:

• Trauma
• Post-splenectomy
• Hemorrhage resulting in iron deficiency
• Various chronic inflammatory diseases

Thrombocytopenia:

• Decreased platelet production
• Toxic drugs and chemicals
• Uremia, septicemia, endotoxemia
• Infections:
  • Viral (infectious canine hepatitis, CPV, distemper, equine infectious anemia)
  • Rickettsial (ehrlichiosis)
  • Bacterial (leptospirosis, salmonellosis)
  • Protozoal (babesiosis)
  • Fungal (histoplasmosis, candidiasis)
• Bone marrow infiltration (neoplasia, leukemias)
• Hyperadrenocorticism
  

2. MPV (Mean Platelet Volume)

Represents the average platelet size and is expressed in femtoliters (fL).

• Increased MPV: May indicate regenerative processes, as the bone marrow produces larger platelets; also seen in some inflammatory and infectious conditions.
• Decreased MPV: Observed in bone marrow failure and disorders of platelet production.

Resources:

Thrall, M. A., Weiser, G., Allison, R. W., & Campbell, T. W. (2012). Veterinary Hematology and Clinical Chemistry. John Wiley & Sons.

Harvey, J. W. (2012). Veterinary Hematology: A Diagnostic Guide and Color Atlas. Elsevier Health Sciences.

Stockham, S. L., & Scott, M. A. (2008). Fundamentals of Veterinary Clinical Pathology. Blackwell Publishing.

Kaneko, J. J., Harvey, J. W., & Bruss, M. L. (2008). Clinical Biochemistry of Domestic Animals. Academic Press.

Jain, N. C. (1993). Essentials of Veterinary Hematology. Lea & Febiger.