Variations of leukocyte count occur in many infectious, hematologic, inflammatory, and neoplastic diseases.
Therefore, laboratory evaluation of almost all patients begins with the examination of the patient’s blood for total leukocyte count and examination of the peripheral blood smear for the differential leukocyte count as well as blood cells picture. Usually, some clinical interpretations may be made from the total leukocyte count and differential leukocyte count.
From total leukocyte count and differential leukocyte count, the absolute count of each leukocyte type can be calculated. The absolute leukocyte count provides a more accurate picture than the differential leukocyte count. (For example, a chronic lymphocytic leukemia patient has a total leukocyte count of 100 x 103 cells µI and a differential leukocyte count of 7 percent neutrophils and 90 percent lymphocytes. By looking at the differential leukocyte count of 7 percent neutrophils alone one get the impression of very low neutrophils in this patient But if the absolute count of neutrophils is calculated, one will be surprised to see a normal neutrophil number in this patient.
Absolute neutrophil count = Total leukocyte count x neutrophil percent
=100 x 103 x 7/100 = 7 x 103 = 7000 / µl
At times of acute bacterial infection enormous numbers of neutrophils are required. Therefore, large number of neutrophils is released from bone marrow to cope with this requirement. Consequently, the number as well as the percentage of neutrophils in the blood increases several fold. Hence, an increase in total leukocyte count with increase in percentage of neutrophil is taken as an important indication of acute bacterial infection.
The terms leukocytosis and leukopenia indicate increase or decrease in the total number of leukocytes, respectively. Increase in numbers of neutrophils, eosinophils, lymphocytes, and monocytes are known as neutrophilia, eosinophilia, lymphocytosis, and monocytosis respectively.
i. Neutrophilia is the increase in peripheral blood absolute neutrophil count, above the upper limit of normal of 7.5 x 109/L (in adults). Bacterial infection is one of the common causes of neutrophilia. Neutrophilia is not a feature of viral infections (However, the development of neutrophila late in the course of a viral infection may indicate the emergence of secondary bacterial infection). There is a storage pool of mature neutrophils in the bone marrow (Such storage pool does not occur for other leukocytes).
In response to stress, such as infections the neutrophils from storage pool are released into circulation, resulting in a rise in total leukocyte count and neutrophil percentage. Moreover, there is increased neutrophil production in the bone marrow. Increased neutrophil production during bacterial infection is usually associated with the entry of less mature neutrophils from bone marrow into blood. This is indicated by the appearance of neutrophils with lesser number of nuclear segmentation in the peripheral blood picture. Band cells may also be seen in the peripheral blood smear.
This is referred to as a ‘shift to the left’. The leukemoid reaction is a reactive and excessive leukocytosis, wherein the peripheral blood smear shows the presence of immature cells (e.g. myeloblasts, promyelocytes, and myelocytes). Leukemoid picture occurs in severe or chronic infections and hemolysis. Another most common change that occurs in neutrophils during infection is the presence of toxic basophilic inclusions in the cytoplasm.
Eosinophilia is an increase in peripheral blood absolute eosinophil count beyond 0.4 x 109/L (in adults). Eosinophilia is usually associated with allergic conditions such as asthma and hay fever and parasitic infections. Eosinophilia may also occur in reactions to drugs.
Decrease in total leukocyte count is known as leukopenia. Reduction in the number of neutrophils is the most frequent cause of leukopenia.
i. Neutropenia is the decrease in peripheral blood absolute neutrophil count below the lower limit of normal of 2 x 109/L (in adults). Neutropenic patients are more vulnerable to infection.
ii. Lymphopenia in adults is the decrease in peripheral blood absolute lymphocyte count below the lower limit of normal of 1.5 x 109/L. Lymphopenia is common in the leukopenic prodromal phase of many viral infections. A selective depletion of helper T lymphocytes (CD4+) with or without absolute lymphopenia occurs in acquired immunodeficiency disease (AIDS).