Andrea Cortese Hassett,
ITxM Diagnostics Chief Science Officer
Many clinicians are
challenged by a patient at risk for bleeding due to the presence of an
acquired or functional platelet disorder. These complex disorders are
difficult to diagnose in part due to the ambiguities in the laboratory
assessment of platelet function. This review will discuss platelet
functional activity and the traditional and newer laboratory methods for
platelet function analysis.
The mechanisms by which
platelets participate in hemostasis are complex. Extensive reviews are
available to detail the mechanisms and pathology of platelet disorders. In
brief, platelet activity can be divided into the following functions:
adhere to damaged blood vessels in a process mediated in part by binding of
von Willebrand factor to the glycoprotein Ib-IX-V complex on the platelet
platelet-to-platelet interaction is initiated by many different agonists,
which bind to specific receptors on the platelet membrane. In vivo,
platelet aggregation is strongly dependent on fibrinogen binding to platelet
activation and aggregation, the contents of platelet granules are secreted
into the exterior environment where they play an important role in the
augmentation and propagation of the hemostatic plug.
4) Activation of
coagulation-platelets provide a procoagulant surface for activated
coagulation protein complexes on their phospholipid membranes.
Platelet function studies
measure and/or monitor the platelet’s ability to adhere and aggregate.
These tests have historically presented a challenge for the clinical
laboratory due to the lack of reliable, accurate and easy-to-perform
Many methods have been used
to measure platelet function. These include bleeding time, aggregometry,
automated functional analyzers, thromboelastography (TEG) , and flow
cytometric monitoring of platelet
activation or adhesion
Platelet aggregationis a measure of the
in vivo ability of platelets to adhere to one another and form the
primary hemostatic plug. It can be performed using either platelet-rich
plasma (PRP) or whole blood. Substances such as collagen, ristocetin,
arachidonic acid, serotonin, ADP, epinephrine, and thrombin stimulate
platelets and hence induce aggregation. Response to these aggregating
agents or agonists, provide a diagnostic signature for platelet function.
Platelet aggregation is
affected by a number of confounding variables. Hemolysis complicates
aggregation measurements since erythrocytes contain ADP; lipemic samples
obscure spectral changes due to platelet aggregation and thrombocytopenia
makes platelet aggregation evaluations difficult to interpret. This test is
laborious and costly and is not readily available to all facilities.
Bleeding time was developed in the hope that
quantifying the length of time a patient bled after a standardized incision
would aid in the diagnosis of platelet disorders and would predict the risk
of bleeding. Unfortunately, it is of limited assistance in evaluating
individual patients, because of low reproducibility, questionable
sensitivity, and unsuitability for serial testing and weak correlation to
bleeding tendency. Despite these shortcomings, the bleeding time remains
widely used in most community and tertiary care centers, although several
alternative technologies for a platelet functional screening test have been
The Platelet Function
Analyzer (PFA-100) (Dade-Behring, Miami, FL) at present is the most
widely used automated analyzer. This system uses membranes pre-coated with
collagen/epinephrine or collagen/ADP to stimulate platelet aggregation. A
blood sample is drawn under vacuum through a capillary that activates the
platelets via shear force. The time it takes to form a platelet plug that
blocks the aperture is measured and known as a closure time. Studies have
demonstrated the utility of the PFA for the detection of acquired platelet
disorders (aspirin induced bleeding) and von Willebrand’s disease. The
interest and use of the PFA-100 is increasing in the clinical laboratory due
to the rapid, cost-effective and clinically relevant information that it
Beaumont, Texas) is arapid platelet aggregation
screening that is designed to determine the percentage of platelet
aggregation in fresh whole blood samples taken during interventional cardiac
procedures. It is a point of care test requiring a hematology analyzer and
measures the change in platelet count due to aggregation of functional
Flow cytometry, long regarded as a research
tool, has been gaining clinical use to study platelet structure and
function. Methods have been developed to evaluate platelet activation and
to diagnose platelet glycoprotein deficiencies. Cost and technically
complexity has limited wide spread clinical acceptance of the method.
Other recent automated
analyzers that have been introduced to evaluate platelet function are the
Xylum Clot Signature Analyzer, the Ultegra RPFA and the HemoStatus test. In
summary these analyzers provide different ways to evaluate platelet function
and hemostasis. Experience with these instruments may provide complementary
data to our existing platelet technologies to assist in the determination of
the overall hemostatic status.
Platelet dysfunction can
lead to a clinical bleeding disorder that is congenital or acquired in
nature. Laboratory evaluation of platelet dysfunction, although
complicated, can be simplified. Prior to any laboratory testing a thorough
medical, family, and drug history is essential in establishing the disorder,
as is the exclusion of other coagulation or fibrinolytic disorders. The
following outlines an approach for evaluating a patient with a suspected
platelet dysfunction and a normal platelet count (the most common scenario).
Platelets have a seminal
role in hemostasis and thrombosis. Platelet dysfunction may be acquired,
inherited, or induced by platelet inhibiting agents. It is clinically
important to assess platelet function to predict a bleeding risk, increase
the understanding of the pathophysiology of platelet disorders and to
provide more definitive patient management.
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