(Study type: Blood from a lavender-top [EDTA] tube; related body system: Circulatory/hematopoietic and immune systems. The specimen should be mixed gently by inverting the tube 10 times. The specimen should be analyzed within 24 hr when stored at room temperature or within 48 hr if stored at refrigerated temperature. If it is anticipated the specimen will not be analyzed within 24 hr, two blood smears should be made immediately after the venipuncture and submitted with the blood sample. Smears made from specimens older than 24 hr may contain an unacceptable number of misleading artifactual abnormalities of the WBCs, such as necrobiotic WBCs.)

WBCs constitute the body’s primary defense system against foreign organisms, tissues, and other substances. The life span of a normal WBC is 13 to 20 days. Old WBCs are destroyed by the lymphatic system and excreted in the feces. Reference values for WBC counts vary significantly with age. WBC counts vary diurnally, with counts being lowest in the morning and highest in the late afternoon. Other variables such as stress and high levels of activity or physical exercise can trigger transient increases of 2 × 10³/microL to 5 × 10³/microL. The main WBC types are neutrophils (band and segmented neutrophils), eosinophils, basophils, monocytes, and lymphocytes. WBCs are produced in the bone marrow. B-cell lymphocytes remain in the bone marrow to mature. T-cell lymphocytes migrate to and mature in the thymus. An increased WBC count is termed leukocytosis, and a decreased WBC count is termed leukopenia.

A total WBC count indicates the degree of response to a pathological process, but a more complete evaluation for specific diagnoses for any one disorder is provided by the differential count. The WBCs in the count and differential are reported as an absolute value and as a percentage. The relative percentages of cell types are arrived at by basing the enumeration of each cell type on a 100-cell count. The absolute value is obtained by multiplying the relative percentage value of each cell type by the total WBC count. For example, on a CBC report, with a total WBC of 9 × 10³/microL and WBC differential with 92% segmented neutrophils, 1% band neutrophils, 5% lymphocytes, and 1% monocytes the absolute values are calculated as follows: 92/100 × 9 = 8.3 segs, 1/100 × 9 = 0.09 bands, 5/100 × 9 = 0.45 lymphs, 1/100 × 9 = 0.1 monos for a total of 9 WBC count. The absolute neutrophil count (ANC) for this patient would be 9 × (0.92 + 0.01) = 8.4.

The WBC count can be performed alone with the differential cell count or as part of the complete blood count (CBC). The WBC differential can be performed by an automated instrument or manually on a slide prepared from a stained peripheral blood sample. Automated instruments provide excellent, reliable information, but the accuracy of the WBC count can be affected by the presence of circulating nucleated red blood cells (RBCs), clumped platelets, fibrin strands, cold agglutinins, cryoglobulins, intracellular parasitic organisms, or other significant blood cell inclusions and may not be identified in the interpretation of an automated blood count. The decision to report a manual or automated differential is based on specific criteria established by the laboratory. The criteria are designed to identify findings that warrant further investigation or confirmation by manual review.

The ANC reflects the number of segmented and band type neutrophils in the total WBC count. It is used as an indicator of immune status because it reflects the type and number of WBC available to rapidly respond to an infection. Neutropenia is a decrease below normal in the number of neutrophils. ANC = Total WBC × [(Segs/100) + (Bands/100)] or total WBC × (% Segs + % Bands). The normal value varies with age, but in general, mild neutropenia is less than 1.5, moderate neutropenia is between 0.5 and 1, and severe neutropenia is less than 0.5. The ANC is helpful when managing patients receiving chemotherapy. It can drive decisions to place a hospitalized patient in isolation in order to protect the patient from exposure to infectious agents. Patients who are aware of their ANC can also make informed decisions in taking actions to avoid exposure to crowds, avoid touching things in public places that may carry germs, or avoid friends and family who may be sick.

Acute leukocytosis is initially accompanied by changes in the WBC count population, followed by changes within the individual WBCs. Leukocytosis usually occurs by way of increase in a single WBC family rather than a proportional increase in all cell types. Toxic granulation and vacuolation are commonly seen in leukocytosis accompanied by a shift to the left, or increase in the percentage of immature neutrophils to mature segmented neutrophils. An increased number or percentage of immature granulocytes, reflected by a shift to the left, represents production of WBCs and is useful as an indicator of infection. Immature neutrophils are called bands and can represent 3% to 5% of total circulating neutrophils in healthy individuals. Bandemia is defined by the presence of greater than 6% to 10% band neutrophils in the total neutrophil cell population. These changes in the white cell population are most commonly associated with an infectious process, usually bacterial, but they can occur in healthy individuals who are under stress (in response to epinephrine production), such as women in childbirth and very young infants. The WBC count and differential of a patient in labor or of an actively crying infant may show an overall increase in WBCs with a shift to the left. Before initiating any kind of intervention, it is important to determine whether an increased WBC count is the result of a normal condition involving physiological stress or a pathological process. The use of multiple specimen types may confuse the interpretation of results in infants. Multiple samples from the same collection site (i.e., capillary versus venous) may be necessary to obtain an accurate assessment of the WBC picture in these young patients.

Neutrophils

Neutrophils are normally found as the predominant WBC type in the circulating blood. Also called polymorphonuclear cells, they are the body’s first line of defense through the process of phagocytosis. They also contain enzymes and pyogenes, which combat foreign invaders.

Lymphocytes

Lymphocytes are agranular, mononuclear blood cells that are smaller than granulocytes. They are found in the next highest percentage in normal circulation. Lymphocytes are classified as B cells and T cells. Both types are formed in the bone marrow, but B cells mature in the bone marrow and T cells mature in the thymus. Lymphocytes play a major role in the body’s natural immune defense system. B cells differentiate into immunoglobulin-synthesizing plasma cells. T cells function as cellular mediators of immunity and comprise helper/inducer (CD4) lymphocytes, delayed hypersensitivity lymphocytes, cytotoxic (CD8 or CD4) lymphocytes, and suppressor (CD8) lymphocytes.

Monocytes

Monocytes are mononuclear cells similar to lymphocytes, but they are related more closely to granulocytes in terms of their function. They are formed in the bone marrow from the same cells as those that produce neutrophils. The major function of monocytes is phagocytosis. Monocytes stay in the peripheral blood for about 70 hr, after which they migrate into the tissues and become macrophages.

Eosinophils

The main function of eosinophils is response to allergy-inducing substances and parasites and phagocytosis of antigen-antibody complexes. Eosinophils have granules that contain histamine used to kill foreign cells in the body. Eosinophils also contain proteolytic substances that damage parasitic worms. The binding of histamine to receptor sites on cells results in smooth muscle contraction in the bronchioles and upper respiratory tract, constriction of pulmonary vessels, increased mucus production, and secretion of acid by the cells that line the stomach. The contents of eosinophilic granules are very effective in neutralizing allergens. However, the substances released by the eosinophils can also damage normal cells in the area where the histamine and other enzymes are released. Eosinophil counts can increase to greater than 30% of normal in parasitic infections; however, a significant percentage of children with visceral larva migrans infestations have normal eosinophil counts.

A nasal smear can be examined for the presence of eosinophils to screen for allergic conditions. Either a single smear or smears of nasal secretions from each side of the nose should be submitted, at room temperature, for Hansel staining and evaluation. Normal findings vary by laboratory, but generally, greater than 10% to 15% is considered eosinophilia, or increased presence of eosinophils.

Basophils

Basophils are found in small numbers in the circulating blood. They have a phagocytic function and, similar to eosinophils, contain numerous specific granules. Basophilic granules contain heparin, histamines, and serotonin. Basophils may also be found in tissue and as such are classified as mast cells. Basophilia is noted in conditions such as leukemia, Hodgkin disease, polycythemia vera, ulcerative colitis, nephrosis, and chronic hypersensitivity states.