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CHAPTER 30 ■ Instrumentation in Hematology 641
related to the concentration o von Willebrand actor, and the number o pulses generated is directly proportional to
the percent normal activity can be obtained rom the stan- the number o cells passing through the sensing zone in a
dard curve. Patient values are determined by comparison to speci c period.
a standard curve, allowing quantitation o percent ristocetin ■ Based on the original ideas o Einstein and physical theo-
co actor activity. ries in the 1950s, laser light was applied to medical and
scienti c instrumentation. Lasers are able to sort the
Platelet Aggregation energy in atoms and molecules, concentrate it, and release
it in power ul waves.
Most platelet aggregation procedures are based on some Flow-cell cytometry is another method that is applied
variation o Born method. Agents such as adenosine diphos- ■ in the study o cells. T e principle o f ow cytometry is
phate (ADP), collagen, epinephrine, snake venom, throm- based on the act that cells can be stained speci cally with
bin, and ristocetin can also be used to aggregate platelets. a f uorescent dye to identi y exact cell types. Laser light is
T e principle o the test is that platelet-rich plasma is
treated with a known aggregating agent. I aggregated, cloud- combined with this method in state-o -the-art instrumen-
tation or cell identi cation and sorting.
iness or turbidity patterns are determined by photometri- Te values that an instrument generates are re erred to as
cally comparing the light transmitted through a suspension ■ parameters. T e simplest units count erythrocytes, leuko-
o aggregated platelets with that o a suspension o nonag- cytes, and platelets. T e most sophisticated instruments
gregated platelets using an aggregometer. T e curve that is generate many additional parameters.
obtained can be used to assess platelet unction.
■ Some instruments are based on a variety o principles
Prim ary Response including the electrical impedance principle and the opti-
cal principle o laser scatter technology.
Primary response is the reversible aggregation o platelets by In addition to numerical outputs, the larger instruments
the aggregating agent. T e appearance o a biphasic reaction, ■ are capable o generating graphic displays o the requency
showing both primary and secondary response, can occur distributions o erythrocytes, leukocytes, platelets, and
or some agonists at low concentrations.
histograms.
Secondary Response ■ Quality control systems, such as Levey-Jennings charts,
are also generated by the larger instruments.
Secondary response is the result o enhancement o the ini-
tial aggregation process caused by the release o endogenous
ADP and the ormation o thromboxane A . T e secondary Whole Blood Cell Analysis
2
response is irreversible. ■ A signi cant innovation is automated ront-end (preana-
lytical [preexamination]) instrumentation/robotics and
Newer Automation total work cells linked by a track or conveyor.
Te PFA-100 (Siemens Healthcare Diagnostics) is an auto- ■ Automated analyzers now orm the backbone o clinical
mated system that incorporates a high shear f ow system laboratories both large and small. Smaller analyzers are
to simulate the in vivo hemodynamic conditions o plate- commonly used in S A labs, reestanding clinics, phy-
let adhesion and aggregation as encountered at a vascular sicians’ o ces, and small hospital laboratories. Larger
lesion. T e system evaluates the ability o platelets to occlude and more complex systems are used in larger clinical and
an aperture in a biochemically active membrane. Results are research laboratories.
reported as closure time (C ). T is instrument o ers several ■ Te degree o instrumental sophistication is requently
advantages over traditional aggregometry because it assesses described by the number o parameters that the instru-
multiple acets o primary hemostasis—adherence, activa- ment generates. T e term parameter is a statistical term
tion, and aggregation. that re ers to any numerical value that describes an entire
population. Parameter should be clearly distinguished
rom the term sample, which is a subset o a popula-
CHAPTER HIGHLIGHTS tion. Any numerical value describing a sample is called
a statistic.
Instrumental Principles
■ Smaller hematology instruments measure erythrocytes
■ Various principles o cell counting are used in instrumen- (RBCs), leukocytes (WBCs), and platelets. Entry-level
tation—electrical impedance, and optical. hematology instruments generate eight measured or cal-
■ Te impedance principle is based on the detection and culated parameters (WBC, RBC, Hgb, Hct, MCV, mean
measurement o changes in electrical resistance produced corpuscular hemoglobin [MCH], mean corpuscular
by a particle as it passes through a small aperture. hemoglobin concentration [MCHC], and platelets).
■ In the optical principle, the degree o scatter and the ■ Computerized systems generally f ag high or low patient
amount o light reaching the sensor depend on the volume results. Additional basic parameters include erythrocyte
o the cell. T e volume o each cell is proportional to the morphology in ormation expressed as RDW, MPV, or leu-
intensity o the orward scatter o light. In both systems, kocyte histogram di erential.
