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Figure 7-17.—Development of blood cells. |
12. When all the cells in the 4 fields have been
counted, multiply the count by 50. This will
give you the total number of white cells per
cubic millimeter of blood.
DIFFERENTIAL WHITE BLOOD
CELL COUNT
A total white blood cell count is not necessarily
indicative of the severity of a disease, since some
serious ailments may show a low white cell count. For
this reason, a differential white cell count is performed.
A differential white cell count consists of an
examination of blood to determine the presence and
the number of different types of white blood cells. This
study often provides helpful information in
determining the severity and extent of an infection,
more than any other single procedure used in the
examination of the blood.
The role of white blood cells, or leukocytes, is to
control various disease conditions. Although these
cells do most of their work outside the circulatory
system, they use the blood for transportation to sites of
infection.
Five types of white cells are normally found in the
circulating blood. They are
· eosinophils,
· basophils,
· neutrophils,
· lymphocytes, and
· monocytes.
Cell Identification
To perform a differential white cell count, you
must be able to identify the different types of white
cells. The ability to properly identify the different
types of white cells is not difficult to develop, but it
does require a thorough knowledge of staining
characteristics and morphology (the study of the form
and structure of organisms). This knowledge can be
gained only by extensive, supervised practice.
To acquaint you with the developmental stages of
each type of leukocyte, a colorized illustration
(fig. 7-17) has been provided. This illustration also
displays the developmental stages of the red blood
cell (erythrocyte) and the blood platelet cell
(thrombocyte). To further assist you, identifying
characteristics of each type of leukocyte as they
appear on a stained blood smear will be covered in the
following sections.
Laboratories use a blood smear to obtain a
differential white cell count. To prepare a blood
smear, a blood specimen is spread across a glass slide,
stained to enhance leukocyte identification, and
examined microscopically. Material requirements
and the step-by-step procedure for performing a blood
smear will be covered later in this chapter.
NEUTROPHILS.—Neutrophils account for the
largest percentage of leukocytes found in a normal
blood sample, and function by ingesting invading
bacteria. On a stained blood smear, the cytoplasm of a
neutrophil has numerous fine, barely visible
lilac-colored granules and a dark purple or reddish
purple nucleus (see figure 7-17). The nucleus may be
oval, horseshoe, or “S”-shaped, or segmented
(lobulated). Neutrophils are subclassified according
to their age or maturity, which is indicated by changes
in the nucleus. The subclassifications for neutrophilic
cells are metamyelocyte, band, segmented, and
hypersegmented.
Neutrophilic Metamyelocyte.—A neutrophilic
metamyelocyte, also called a “juvenile” cell, is the
youngest neutrophil generally reported. The nucleus
is fat, indented, and is usually “bean”-shaped or
“cashew nut”-shaped (fig. 7-17).
Neutrophilic Band.—A neutrophilic band,
sometimes called a “stab” cell, is an older or
intermediate neutrophil. The nucleus has started to
elongate and has curved itself into a horseshoe or
S-shape.
As the band ages, it matures into a
segmented neutrophil (fig. 7-17).
S e g m e n t e d N e u t ro p h i l . — A s e g m e n t e d
neutrophil is a mature neutrophil. The nucleus of a
segmented neutrophil is separated into two, three,
four, or five segments or lobes (fig. 7-17).
7-19
Example:
25 cells in field #1
23 cells in field #2
26 cells in field #3
26 cells in field #4
100 total cells in all fields
Multiply:
100 x 50
= 5,000
Total white cell count = 5,000 cells/mm3
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