CLINICAL LAB SCIENEC

340
ESSENTIALS OF CLINICAL LABORATORY SCIENCE
In the Lab
Safety Alert
To reduce the risk of
transmission of bloodborne
diseases such as AIDS and
hepatitis B, wear protective
gloves when handling other
peoples’ blood, dispose of all
blood-contaminated materials
in the provided containers,
and clean up thoroughly when
finished.
HEMATOLOGY AND COAGULATION PROCEDURE #3
Determining Hematocrit
Principles
The number of erythrocytes in the blood must be high enough to carry sufficient
oxygen to the peripheral tissues. However, if the number of erythrocytes becomes
elevated, as occurs in some disease states, the blood viscosity (thickness or
stickiness) may be increased. This will adversely affect the heart’s
ability to circulate the blood throughout the body.
A simple test called the hematocrit (Hct) is used to determine the percent
of formed cells in whole blood. Normally, 99% of all the cells in the
blood are RBCs; the remaining 1% includes WBCs and platelets. The term
“hematocrit” (Hct) comes from the Greek “hemato” meaning “blood” and
“crit” meaning “to judge.” Another name for the Hct is PCV (packed cell
volume), or simply “crit.” The test provides an estimate of the adequacy
of numbers of RBCs required to carry oxygen to the cells and organs of
the body.
For this test, a fresh sample of blood is introduced into a capillary tube
coated with heparin to prevent clotting. The end of the tube is sealed with
putty, and the tube is spun at a rapid rate in a special centrifuge called a
microhematocrit centrifuge (Figure 13-8A) to compact the cells into the end
of the tube, where the percentage of RBCs will be determined.
Hematocrit Calculation
The hematocrit (Hct) determination is actually a separation of solids (blood cells) from
the liquid portion (plasma) of the blood. A centrifuge uses centrifugal force to press the
RBCs to the bottom of the capillary tube, leaving the liquid part or plasma overlying
the RBCs.
Hct values are determined in two ways. They may be performed by calculations
from an automated cell counter or by centrifuging a small amount of blood in a
capillary tube. The manual method for measuring the Hct requires a reader, of which
there are various versions (Figure 13-8B).
The straw-colored supernatant is the plasma, the RBCs sink to the bottom,
and the WBCs are seen as a thin buffy coat, if the WBC count is normal, at the
top of the RBC column. WBCs are not included in the Hct. (Note that the WBC
count might be quite high in a patient with a severe infection or leukemia.) Figure
13-9 shows a centrifuged capillary tube with blood kept from clotting by a thin
layer of heparin (an anticoagulant) on the inside of the tube. The percentage of
the total RBCs represented by the packed cells is the percentage of RBCs in the
whole blood specimen. Normal Hct values for males are 40% to 54%, and those for
females, 37% to 47%.
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