White blood cell count and differential

Encyclopedia of Nursing and Allied Health, 20050229 by Victoria E. DeMoranville

Definition

A white blood cell (WBC) count determines the concentration of white blood cells in the patient's blood. A differential determines the percentage of each of the five types of mature white blood cells.

Purpose

This test is included in general health examinations and to help investigate a variety of illnesses. An elevated WBC count occurs in infection, allergy, systemic illness, inflammation, tissue injury, and leukemia. A low WBC count may occur in some viral infections, immunodeficiency states, and bone marrow failure. The WBC count provides clues about certain illnesses, and helps physicians monitor a patient's recovery from others. Abnormal counts which return to normal indicate that the condition is improving, while counts that become more abnormal indicate that the condition is worsening. The differential will reveal which WBC types are affected most. For example, an elevated WBC count with an absolute increase in lymphocytes having an atypical appearance is most often caused by infectious mononucleosis. The differential will also identify early WBCs which may be reactive (e.g. a response to acute infection) or the result of a leukemia.

Precautions

Many medications affect the WBC count. Both prescription and non-prescription drugs including herbal supplements should be noted. Normal values for both the WBC count and differential are age related.

Sources of error in manual WBC counting are largely due to variance in the dilution of the sample and the distribution of cells in the chamber, and the small number of WBCs that are counted. For electronic WBC counts and differentials, interference may be caused by small fibrin clots, nucleated RBCs, platelet clumping, and unlysed RBCs. Immature WBCs and nucleated RBCs may cause interference with the automated differential count. Automated cell counters may not be acceptable for counting white blood cells in other body fluids especially when the number of WBCs is less than 1000/µL or when other nucleated cell types are present.

Description

White cell counts are usually performed using an automated instrument, but may be done manually using a microscope and a counting chamber especially when counts are very low, or the person has a condition known to interfere with an automated WBC count. An electronic WBC count is based upon the principle of impedance. The red blood cells are lysed using a detergent in the counting diluent. As the cells move one at a time through a counting aperture, they displace electrolyte in the diluent causing a voltage pulse. The magnitude of the voltage pulse is dependent upon size which allows the instrument to discriminate between different types of WBCs.

An automated differential may be performed by an electronic cell counter or by an image analysis instrument. The automated electronic cell counter uses a combination of impedence measurement and other means such as radio frequency conductance and angular light scattering to differentiate between closely related WBCs. Image analysis systems use morphometric and densitometric programs to distinguish the cells which are photographed from a stained slide by a digital color camera. When the electronic WBC count is abnormal or a cell population is flagged, meaning that one or more of the results is atypical, a manual differential is performed. The WBC differential is performed manually by microscopic examination of a blood sample that is spread in a thin film on a glass slide. The film is air-dried and stained with Wright stain, a polychromatic stain consisting of buffered solutions of methylene blue and eosin. Acidic structures such as DNA take up the basic methylene blue dye, while basic proteins, such as hemoglobin, take up the acidic eosin dye. White blood cells are identified by their size, the shape and texture of the nuclear chromatin, cytoplasmic and nuclear staining, and the presence and color of granules in the cytoplasm.

The manual WBC differential involves a thorough evaluation of a stained blood film. In addition to determining the percentage of each mature white blood cell, the following tests are preformed as part of the differential:

Evaluation of RBC morphology is performed. This includes grading of the variation in RBC size (anisocytosis) and shape (poikioocytosis); reporting the type and number of any abnormal RBCs such as target cells, sickle cells, stippled cells, etc.; reporting the presence of immature RBCs (polychromasia); and counting the number of nucleated RBCs per 100 WBCs.

An estimate of the WBC count is made and compared to the automated or chamber WBC count. An estimate of the platelet count is made and compared to the automated or chamber platelet count. Abnormal platelets such as clumped platelets or excessively large platelets are noted on the report.

Any immature white blood cells are included in the differential count of 100 cells, and any inclusions or abnormalities of the WBCs are reported.

WBCs consist of two main subpopulations, the mononuclear cells and the granulocytic cells. Mononuclear cells include lymphocytes and monocytes. Granulocytes include neutropohils (also called polymorphonuclear leukocytes or segmented neutrophils), eosinophils, and basophils. Each cell type is described below: