Interpretation of the Hemogram: Introduction, White Cells, Red Cells, Platelets

Introduction

Hemograms consist of both quantitative data (total cell counts, differential cell counts, red cell indices, etc.) and qualitative data (blood film morphology). Proper interpretation depends on the integration of both.
Proper interpretation also depends upon the development of a systematic approach. For both quantitative and qualitative data, we recommend evaluation of white cells first, followed by red cells, and then platelets.
For all cell compartments, interpretation can be guided by asking and answering a series of well-designed questions.

White Cells

Overview (Table 11-1)

> Quantitative data includes total white cell count and differential count

> Qualitative data is white cell morphology

> Key questions include:

• Is there evidence of inflammation?
• Is there evidence of stress?
• Is there evidence of tissue necrosis?
• Is there evidence of systemic hypersensitivity?
• If inflammatory, can the response be classified as acute, chronic, or overwhelming?
• Is there evidence of systemic toxemia?

Is there evidence of inflammation?

> Persistent eosinophilia, monocytosis, and a neutrophilic left shift (increased numbers of immature neutrophils), alone or in combination, suggest inflammation.

> Total white cell count merely reflects balance between marrow production and tissue utilization; in inflammation, total white cell counts may be low, normal, or high.

> Absolute neutrophilias of greater than 25,000/μl are also suggestive of inflammation.

Is there evidence of stress (high circulating levels of glucocorticords)?

> Stress typically results in mild lymphopenia (lymphocyte counts between 750/μl and 1500/μl).

> Eosinopenia, mild neutrophilia, and mild monocytosis may also be present but are less consistent and nonspecific.

Is there evidence of tissue necrosis?

> Monocytosis indicates tissue necrosis and demand for phagocytosis.

Monocytosis can occur with acute or chronic inflammation or necrosis.

Is there evidence of systemic hypersensitivity?

> Persistent eosinophilia and/or basophilia is an indicator of systemic hypersensitivity.

> Causes include:

• Parasitic diseases with a systemic component, eg., heartworms, flea bite dermatitis
• Allergic tracheobronchitis in dogs (pulmonary infiltrates with eosinophils)
• Feline asthma
• Allergic gastroenteritis
• Systemic mastocytosis
• Disseminated eosinophilic granuloma complex in cats
• Parasitic disease confined to the intestinal tract (eg., whip worms) does not cause eosinophilia!!

Can the inflammatory response be classified as acute, chronic, or overwhelming?

> In many cases, inflammatory leukograms cannot be further classified.

> In other cases, the differential cell count is typical of acute, chronic, or overwhelming inflammation.

> These typical patterns reflect changes in leukocyte kinetics, or the balance between white cell production in the marrow and white cell utilization in the tissues. These changes are controlled by chemotactic factors and cytokines.

> The typical acute inflammatory leukogram is characterized by neutrophilia with increased band cells (a regenerative left shift), lymphopenia, and variable monocytosis.

• Neutrophilia reflects a large bone marrow storage pool in dogs and cats and the movement of larger numbers of neutrophils from marrow into blood than are moving from blood into tissues.
• The left shift suggests depletion of the marrow storage pool of neutrophils with the subsequent recruitment of younger cells into circulation.
• The lymphopenia reflects stress, a common accompaniment of acute inflammatory processes.
• When present, the monocytosis reflects demand for phagocytosis / tissue necrosis.

> There are two patterns typical of chronic inflammation:

• Marked leukocytosis (50,000 - 120,000/μl) with marked neutrophilia and left shift, neutrophil toxicity, and monocytosis.

• Most commonly seen with severe focal suppurative lesions
• Usually accompanied by the anemia of inflammatory disease and hyperglobulinemia

• Normal to slightly elevated white cell count characterized by normal to slightly elevated neutrophil counts, no left shift, normal lymphocyte counts, and monocytosis.

• The normal to slightly elevated neutrophil count reflects a new balance between marrow production and tissue demand. This balance results from expanded production of neutrophils by the bone marrow in response to cytokines (growth factors) released at the tissue site of injury.
• The lack of left shift reflects the fact that marrow production of neutrophils has expanded to meet increased tissue demand.
• The normal lymphocyte count reflects the counterbalancing effects of stress and antigenic stimulation on lymphocyte numbers.
• Monocytosis reflects demand for phagocytosis/tissue necrosis.

> The typical overwhelming inflammatory response is characterized by reduced neutrophil numbers, a left shift, lymphopenia, and variable monocytosis.

• Reduced numbers of neutrophils suggest inability of marrow production to keep pace with tissue demand.
• Left shift indicates depletion of marrow neutrophil storage pools.
• Lymphopenia reflects stress.
• When present, monocytosis indicates tissue necrosis/demand for phagocytosis.

Is there evidence of systemic toxemia?

> The presence of toxic neutrophils on the blood film indicates systemic toxemia (see neutrophil morphology)

• Systemic toxemia is most commonly associated with bacterial infections.
• However, other causes, such as extensive tissue necrosis, must also be considered.

Red Cells

Overview

> Quantitative data includes red cell count, hemoglobin, hematocrit, red cell indices (MCV, MCHC), and total protein.

• Red cell count, hemoglobin, and hematocrit are all measures of red cell mass.
• Total protein provides information about state of hydration. Elevations most commonly result from dehydration which can also falsely elevate indicators of red cell mass.

> Qualitative data is red cell morphology determined from the blood film.

> Key questions include:

• Is red cell mass increased (polycythemia), decreased (anemia), or normal?
• If decreased, is anemia regenerative or nonregenerative?
• If regenerative, is the mechanism blood loss or hemolysis?
• If nonregenerative, can the mechanism be determined without bone marrow evaluation?
• If red cell mass is increased, is the polycythemia relative or absolute?
• If polycythemia is absolute, is it primary or secondary?

Is red cell mass increased, decreased, or normal?

> Answered by evaluating the indicators of red cell mass.

If decreased, is the anemia regenerative or nonregenerative (Fig. 11-1)?

> Evaluating the blood film is the critical first step in recognizing regenerative anemias. Increased numbers of polychromatophilic erythrocytes on the blood film suggests red cell regeneration.

> Regeneration is confirmed by doing absolute reticulocyte counts. In dogs and cats, absolute reticulocyte counts of greater than 80,000/μl indicate regeneration.

Figure 11-1. Interpretive approach to the evaluation of anemia.

If regenerative, is the mechanism blood loss or hemolysis?

> History, signs, and physical exam are key to differentiation. Most causes of blood loss will be recognized in this way.

> Hemoglobinemia or hemoglobinuria indicate hemolysis.

> Very high reticulocyte counts (>200,000/μl) are highly suggestive of hemolysis.

> Where hemolysis is suspected, red cell morphology should be scrutinized for abnormal red cells which are characteristic of certain hemolytic disorders. These are described and illustrated elsewhere (see Erythrocytes: Overview, Morphology, Quantity) but include:

• Spherocytes
• Heinz bodies
• Schistocytes
• Etiologic agents (Haemobartonella, Babesia)
• Ghost cells
• Eccentrocytes

If nonregenerative, can the mechanism be determined without bone marrow evaluation?

> The anemia of inflammatory disease is the most common anemia of dogs and cats and can be presumptively diagnosed from the hemogram. Characteristics include:

• Mild to moderate normocytic normochromic anemia
• An inflammatory leukogram

> Iron deficiency causes a characteristic microcytic hypochromic nonregenerative anemia which can be presumptively diagnosed from hemogram data and blood films.

> Megaloblastic anemias (nuclear maturation defect anemias) often have occasional giant red cells (macrocytes) in circulation. Megaloblasts (see Erythrocytes: Overview, Morphology, Quantity) may also be present on blood films. Marrow confirmation is required.

> Myelofibrosis of the bone marrow causes nonregenerative anemia with the following characteristics:

• Poikilocytosis with dacryocytes and ovalocytes
• Leukopenia
• Variable platelet response. Marrow histopathologic confirmation is required.

 > Nonregenerative anemias characterized by large numbers (>10/100 WBC counted) of nucleated red cells on blood films in the absence of polychromasia (an inappropriate nucleated red cell response) indicates bone marrow stromal damage. Causes are most likely:

• Lead poisoning in dogs
• FeLV infection in cats

> All other nonregenerative anemias have nonspecific hemogram findings and can only be further assessed via bone marrow evaluation.

If red cell mass is increased, is polycythemia relative or absolute?

> Relative polycythemia (due to dehydration) is the most common form. It is characterized by:

• Increased red cell mass
• Increased total protein
• Serum chemical indicators of dehydration

> When relative polycythemia is ruled out, the remaining cases are absolute polycythemias.

If absolute, is polycythemia secondary or primary?

> Secondary polycythemia is associated with (caused by) a number of other diseases including:

• Cardiovascular disease
• Pulmonary disease
• Renal disease
• Renal neoplasms (primary or metastatic)
• Cushing's disease

> In the absence of such an underlying cause, polycythemia is considered to be primary (due to the myeloproliferative disorder polycythemia vera). Polycythemia vera is characterized by:

• Normal tissue oxygenation (normal arterial blood gas)
• Normal erythropoietin levels

Platelets

Overview

> The quantitative platelet test is platelet number.

> The qualitative platelet test is platelet morphology.

> Key questions include:

• If there is increased platelet count (thrombocytosis), is it reactive or primary?
• If there is a decreased platelet count (thrombocytopenia), can the mechanism be determined?
• If the platelet count is normal but there is evidence of bleeding, could it be the result of dysfunctional platelets (thrombocytopathy)?
• Is platelet morphology abnormal?

If there is thrombocytosis, is it reactive or primary?

> Reactive thrombocytosis is seen secondarily with:

• Splenectomy
• Excitement
• Exercise
• Fractures
• High circulating glucocorticoid levels
• Post-blood loss (24 hours or more)
• Myelofibrosis
• Iron deficiency anemia

> Primary thrombocytosis is seen as a distinctive form of platelet leukemia or in association with other myeloproliferative disorders.

If there is a thrombocytopenia, can the mechanism be determined?

> Consumptive thrombocytopenias are associated with inflammation, DIC, and infectious diseases such as Ehrlichiosis and other tick- borne diseases. Features include:

• Inflammatory leukogram
• Mild to moderate thrombocytopenia (platelet counts generally greater than 50,000)
• Schistocytes (in dogs)
• Normal numbers of marrow precursors

> Sequestration thrombocytopenias are associated with hepatosplenomegaly

> Hypoproliferative thrombocytopenias are associated with reduced numbers of marrow precursors.

> Destructive thrombocytopenias are immune-mediated. They may occur alone or in combination with immune mediated hemolytic disease. Common features include:

• Marked thrombocytopenia (<50,000/μl)
• Normal to increased numbers of marrow precursors

If platelet count is normal but there is evidence of bleeding, could it be the result of thrombocytopathy?

> First rule out other causes of bleeding

• Trauma
• Coagulation defects (activated partial thromboplastin time, APTT, and prothrombin time, PT, are normal)
• DIC - fibrinogen and fibrin split products are also within normal

> Run buccal mucosal bleeding time (BMBT)

• If prolonged, consider thrombocytopathy as a possible (likely) cause

Is platelet morphology abnormal?

> The presence of significant numbers of small platelets (microplatelets) suggests the early phase of a possible immune- mediated thrombocytopenia.

> Platelet anisocytosis characterized by significant numbers of enlarged platelets (macroplatelets) suggests increased marrow production of platelets. Commonly seen in responsive thrombocytopenias and regenerative anemias.

> Poorly granulated platelets with a few large granules suggest developmental abnormalities. Evaluate bone marrow.

> Excessively large granules in platelets should be examined as potential inclusions such as those associated with Ehrlichia platys.