Clinical Laboratory - Hematology

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Complete Blood Count (CBC)

Laboratory evaluation and Differential Diagnosis of Anemia, Erythrocytosis, Leukopenia, Leukocytosis, Thrombocytopenia, and Thrombocytosis


Anemia, defined as decreased number of red blood cells or hemoglobin concentration in the blood, may be due to a wide variety of pathologic processes, including:

Accelerated RBC loss from bleeding, hemolysis, extrinsic abnormalities, and sequestration in the spleen due to enlargement and increased splenic activity. Reticulocytes, are precursors to the mature RBC, and the "retic count" is high when RBCs are being consumed peripherally and the marrow is reacting normally to increase production.

Decreased RBC production results from nutritional deficiencies, chronic disease states, and loss of bone marrow function (metastatic disease, aplastic anemia). The reticulocyte count is low with decreased marrow erythropoiesis.

Abnormal red cell production results from hemoglobinopathies, such as Alpha and Beta Thalessemia.

Anemias are generally classified as:

  • Microcytic
  • Macrocytic
  • Normocytic


An increase in RBC mass is called polycythemia. The hemoglobin and hematocrit are high. This is classified as:

Polycythemia vera: a myeloproliferative process most often seen in older adults.

Secondary polycythemia may arise due to:

  • Paraneoplastic process (e.g., erythropoietin made by a cancer such as a renal cell carcinoma).

  • Chronic hypoxemia (COPD, living at high altitude).

  • Relative polycythemia is due to hemoconcentration from dehydration. Other parameters such as serum creatinine or potassium or sodium may appear abnormally elevated.

Leukopenia and Leukocytosis

Neutropenia is a decrease in the absolute neutrophil count. Pathologic processes that result in neutropenia include processes that decrease production: aplastic anemia, marrow toxins such as drugs, marrow infiltration by infections or metastatic carcinomas, leukemias, radiation therapy, chemotherapy. Disorders that increase neutrophil destruction: splenomegaly with hypersplenism, infection, and immune destruction.

Neutrophilia is an increase in the absolute neutrophil count; it can be increased transiently with stress and exercise by a shift of neutrophils from the marginating pool to the circulating pool. Pathologic processes that result in neutrophilia include: infection, tissue destruction or necrosis (infarction, burns, neoplasia), and hematologic disorders (leukemias, myeloproliferative disorders).

Lymphopenia is a decrease in the number of circulating lymphocytes may be seen with: immunodeficiency syndromes, corticosteroid therapy, cancer, radiation therapy, and chemotherapy.

Lymphocytosis is an increase in the number of circulating lymphocytes may normally be observed in infants and young children. Pathologic processes with lymphocytosis may include: acute infections, infectious mononucleosis, with "atypical" lymphocytosis, viral infections, and leukemias.

Eosinophilia is an absolute increase in the number of circulating eosinophils that most often occurs with allergic reactions and parasitic infestations. It also occurs with hematologic malignancies: chronic myeloproliferative disorders and Hodgkin lymphoma.

Basophilia is an absolute increase in the number of circulating basoophils that most often occurs with chronic myeloproliferative disorders.


Thrombocytopenia is an absolute decrease in circulating platelets. It can be due to marrow failure of megakaryopoiesis. It may also result from peripheral blood consumption of platelets, particularly microangiopathic anemias such as DIC and TTP.

Thrombocytosis is an absolute increase in circulating platelets. It is most often associated with myeloproliferative disorders, in which case the platelets are abnormal and do not work properly and may lead to either thrombosis or bleeding.

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