Principles Of Platelet Aggregation - Chrono-Log 490 4+4 Instruction Manual

Defective liberation of arachidonic acid from the
platelet membrane
Deficiency of the enzyme cyclo-oxygenase
Deficiency of the enzyme thromboxane
synthetase
Secretion defects with normal granule contents and
o
normal Arachidonic Acid metabolic pathway
Defective cytosolic calcium mobilization
o
Defective early responses -myosin light chain
o
phosphorylation; phosphatidylinositol metabolism
Defective or Blocked Receptors to Specific Agonists (in
o
addition to BSS and thrombasthenia)
Defective response to epinephrine -
myeloproliferative disorders (MPD)
Defective response to collagen
Defective response to U46619 (the stable analog
of thromboxane A2)
Defective Platelet Coagulant Activities - the platelet
o
contribution to and interaction with the coagulation
scheme.
Miscellaneous defects
•
Congenital
o
May-Heggelin Anomaly
Down's Syndrome
Thrombocytopenia with absent radii (TAR
syndrome)
Acquired
o
Uremia
Extracorporeal circulation
PRINCIPLES OF PLATELET AGGREGATION TESTS
Platelets are known to aggregate under a variety of conditions
and in the presence of a number of different reagents. "Platelet
aggregation" is a term used to denote the adherence of one
platelet to another. The phenomenon can be induced by adding
aggregating agents to platelet-rich plasma. Platelet aggregation
depends on the presence of calcium, fibrinogen and one or more
plasmatic factors, and an aggregating agent. Platelet aggregation
will vary with different aggregating agents and concentrations.
For optical aggregometry, ADP, epinephrine, collagen and
ristocetin are used extensively for screening purposes and provide
the most immediate information for basic diagnostic
18
considerations.
The selection of these reagents has some basis in theory. Both
ADP and epinephrine (adrenaline) are contained within the
platelet in storage organelles and are released from the platelet
during formation of the primary hemostatic plug and may thereby
induce further platelet aggregation.
platelet response to these reagents has proven to be of help in
determining the nature of a patient's bleeding disorder.
Collagen, on the other hand, is not contained in the platelet but is
found in the supporting connective tissue of the blood vasculature
and is considered to be the first aggregating or pro-coagulant
factor that the platelet encounters following vascular trauma.
Hence, in-vitro study of the platelet response to collagen has
assumed considerable importance.
Other reagents such as thrombin
arachidonic acid, ristocetin, Bovine Factor VIII, and serotonin have
also been used to study platelet response for more specific
purposes.
Platelet aggregation is the most useful in-vitro test of platelet
function presently available. It is a diagnostic tool, which can
Document # 49044IM1
Revision 7.5
Dated February 16, 2017
3 Consequently, in-vitro
4,12
3 , the calcium ionophore A23187,
provide insight that is difficult or impossible to obtain by other
techniques, thus aiding in patient diagnosis and proper selection
of treatment or therapy. Experience with this technique has
delineated a spectrum of inherited and acquired platelet
dysfunctional states.
Platelet aggregation is clinically significant in the detection and
diagnosis of acquired or congenital qualitative platelet defects.
The platelet's ability or inability to respond to particular
aggregating reagents is the basis for differentiating platelet
dysfunctions 6 as shown in the table below:
AGGREGATION STUDIES ON SELECTED PLATELET FUNCTION
DEFECT Aggregation
Thrombasthenia Decreased
Thrombopathia or
Thrombocytopathy (1st phase)
von Willebrand's
Disease
Non-steroidal,
Anti-inflammatory
(1st phase)
drugs
Optical Aggregation Tests
In-vitro platelet aggregation is an effort to characterize the in-vivo
ability of the platelets to form the primary hemostatic plug.
Platelets in a suspension of plasma are isolated from an anti-
coagulated blood sample by a relatively low centrifugal force
centrifugation. This material is known as platelet rich plasma
(PRP). Platelet poor plasma (PPP) is prepared by centrifuging the
blood sample at a relatively high force.
The Born type aggregometer or optical aggregometer is a fixed
wavelength spectrophotometer with a sample chamber (or
chambers) heated to 37°C. Provision is made for stirring of the
sample because platelet to platelet contact is necessary to the
determination of in vitro platelet aggregation.
The CHRONO-LOG® sample chambers are designed so that a
beam of infra-red light shines through two cuvettes, one
containing PRP (the sample) and one containing PPP (the
reference). Silicon photodiodes detect the light able to pass
through the samples: PRP is arbitrarily considered to be 0% light
transmission or 0% aggregation; PPP is considered to be 100%
light transmission or 100% aggregation. The difference in light
transmission outputs from the photodiodes is transferred to
recording devices.
The twin well, dual infra-red beam design ensures reproducibility.
Each channel has a separate well for the test (PRP or washed
platelets; O% light transmission) and corresponding reference
(PPP or buffer; 100% light transmission) samples. A SELECT/SET
switch allows the use of either a separate or a common reference
sample. The optical aggregation output is proportional to the
continuously measured difference in light transmission between
the test and reference samples. Pressing a single pushbutton sets
the 0% and 100% light transmission baselines. With the sensitivity
of the twin-well dual-beam detection system, a count difference
9
of only 75 x 10 /L between the test and reference sample is
4
DEFECTS
Platelet
Platelet Platelet
Aggregation
Aggregation
By ADP by Collagen
Ristocetin
Decreased Normal
Normal
Decreased Normal
Normal Normal Abnormal
Normal
Decreased Abnormal
by