General Approach in General Approach in Investigation of HaemostasisInvestigation of Haemostasis

Platelets aggregation

Studying Platelets Disorders

Numbers CBC

PLT count PLT morphology

Function Bleeding Time (BT) Platelet Aggregation

Whole blood aggregation Platelet-rich plasma aggregation

Platelets contribute to Hemostasis in two main ways:

Primary haemostatic plug : Adhesion Aggregation Secretion

Secondary Haemostatic plug: Procoagulant activities are

generated

Platelet Plug Formation: Adhesion Platelets bind to exposed adhesive

subendothelial connective tissue Collagen vWF Fibronectin

Mechanism components vWF: links PLT to endothelial binding site

PLT receptor GPIb Collagen fibers

Actin contracts & pseudopods form REVERSIBLE

Facilitates activation

Platelets Aggregation Platelet-Platelet interaction Mechanism components

ATP Ionized calcium Fibrinogen Fibrinogen PLT receptor GPIIb/IIIaPLT receptor GPIIb/IIIa

Initial aggregation – REVERSIBLESecondary aggregation – IRREVERSIBLE** = white clot, also know as platelet plug formed.

* The transformation of irreversible aggregated platelets into a mass of degenerative platelet material without membranes is termed viscous metamorphosis. (by platelet lysosomes)

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Why Platelet aggregation testing Evaluation of suspected hereditary and acquired disorders

of platelet function. Platelets normally contain three major types of granules .

The alpha granules contain fibrinogen, PF4, factor V, von Willebrand factor….

The dense granules contain ADP or ATP, calcium and serotonin. lambda granules – similar to lysosomes and contain several hydrolytic

enzymes.

Hereditary platelet function disorders includes Rare defects of adhesion (Bernard Soulier syndrome) Rare defects of aggregation (Glanzmann thrombasthenia) More common defects of secretion (alpha or dense granule

deficiency, aspirin-like defects, or other primary secretion defects). Acquired platelet function disorders are more common

than the hereditary disorders and include drug-induced platelet dysfunction (including aspirin, NSAID’s, clopidogrel, antibiotics, various cardiovascular and psychotropic drugs), uremia, and myeloproliferative disorders.

Using Aggregating agents to induce platelet aggregation or cause platelets to release endogenous ADP, or both.

Platelet aggregation is studied by means of a platelet aggregometer, Used Principle: 1. Photo-optical Method2. luminescence technology (Platelet

Lumiaggregometry)3. Electrical Impedance Method

Principle

AGGREGATING AGENTS

Electrical Impedance Method These types of analyzers may use citrated

whole blood, as the test sample. As platelets aggregate, the coat an

electrode, impeding the electrical current through the analyzer.

http://www.platelet-research.org/3/pfa.htm

Luminescence technology (Platelet Lumi aggregometry)

The lumi-aggregometer may be used to simultaneously measure platelet aggregation and secretion. The instrument records both aggregation and secretion of dense-granule ATP.

The ATP is measured by its reaction with firefly luciferin to give chemiluminescence. The resulting light emission is detected, amplified, and recorded by the instrument.

Performed by using whole blood or PRP. This modification of aggregation is

particularly sensitive to ATP release, and is as sensitive measure of platelet activation.

[A[ + ]B[ → ]AB[ → ]◊Products + ]lightight

Photo-optical Aggregometer (PLT

Aggregometry Using PRP)

The Platelet-rich plasma, which is turbid in appearance, is placed in a cuvettes, warmed to 37°C in the heating block of the instrument, and stirred via a small magnetic bar.

Baseline light transmittance through the platelet-rich plasma is recorded. The addition of an aggregating agent causes the formation of larger platelet aggregates with a corresponding increase in light transmittance, because of a clearing in the platelet-rich plasma. The change in light transmittance is converted to electronic signals and recorded as a tracing by the chart recorder.

Patient Sample – 3.2% citrated WBApproximately 20 ml of blood is needed for a full

aggregation study.Test Sample – PRP ( Platelets count fall (200–600 ×

109/L)Principle – photometry: optical density of PRP

warmed to 37° C is determined before and after the addition of various aggregating agents

IssuesSample quality is criticalFibrinogen levels are importantAgonists must be prepared fresh dailyThrombocytopenia makes result interpretation

difficultComplete patient history is essential

Figure 1 - Platelet-rich plasma in an optical aggregometer. Platelet count is approximately 200 × 109/L, and platelets are maintained in suspension by a magnetic stir bar turning at 1000 rpm. (Courtesy of Kathy Jacobs, Chronolog, Inc., Havertown, PA.)Figure 2 – Five possible phases of PLT aggregation: 1) baseline,

2) agonist addition and shape change,

3) primary wave,

4) secretion, and

5) secondary wave.http://www.platelet-research.org/3/aggregometry.htm

Procedure:1. Switch on aggregometer to warm to 37°C. Select AGGR

TEST2. Select the type of agonist (ADP, AA, RISTO, THR, COLL,

ADR)3. Place 0.27 ml PRP in a test cup and put in the incubation

channels. Put beads into the cups, wait 1 minute to warm.4. Transfer the cups to testing channels. 5. Press PPP, it will appear the status <PPP>6. Add 0.03 ml of agonist to bottom of cuvettes and press

start key then monitor optical density changes for three minutes.

7. Repeat this procedure for each agonist8. Curve Display.

* ADP should placed in ICE.

ADP (at appropriate concentration)Biphasic curve: 1o and 2o waves (requires intact prostaglandin pathway)Note: if ADP is added at too low or too high concentration, it will not get biphasic response

EpinephrineBiphasic curve; requires intact prostaglandin pathway

CollagenLag phase followed by 2o wave only

RistocetinA biphasic however, often only a single broad wave Binds to vWF/GPIb/IX complex and results in agglutination Evaluates adhesion reaction

ThrombinBiphasic curve. Irreversible aggregation only (does not require cyclooxygenase)

Arachidonic acid2o wave only; assesses

cyclooxygenase pathway

Serotonin A primary wave of aggregation

with a maximum of 10% to 30% transmittance followed by disaggregation.

InterpretationInterpretation Platelet aggregation occurs as a two-step process,

known as primary and secondary waves of aggregation.

The primary wave of aggregation is observed when platelets adhere to one another in the presence of an external agent (agonist) such as ADP, epinephrine, or ristocetin.

Secondary aggregation is characterized as the aggregation that occurs after the platelets have been stimulated to secrete the substances contained in their organelles.

It should be noted that some agonists will stimulate primary aggregation and some will stimulate secondary aggregation. Others will stimulate both primary and secondary aggregation, yielding a "biphasic" aggregation curve.

In addition, different concentrations of the same agonist can produce varying patterns of primary and secondary aggregation. For example, Low concentrations of ADP induce biphasic aggregation

(i.e., both a primary and a secondary wave of aggregation); Very low concentrations of ADP (l.5 ug/ml. final

concentration) induce a primary wave followed by disaggregation;

And high concentrations of ADP (10 ug/ml, final concentration) induce a single, broad wave of aggregation“

A biphasic aggregation response to ADP will not be seen in patients with platelet release disorders.

Patients with Glanzrnann's thrombasthenia show incomplete aggregation with ADP regardless of the final concentration.

In patients with severe von Willebrand disease, aggregation to ristocetin is characteristically absent. Decreased to normal aggregation to ristocetin can be seen in patients with mild von Willebrand disease. Correction of the abnormal ristocetin aggregation curves can be seen by the addition of normal, platelet-poor plasma to the patient's platelet-rich plasma.

Abnormal ristocetin-induced platelet aggregation may also occur in patients with 1. Bernard-Soulier syndrome, 2. Platelet storage pool defects3. Idiopathic thrombocytopenia purpura (ITP).

Glanzmann thrombastheniao Normal PLT count, but abnormal clot retraction

o Absence of secondary aggregation to ADP, epinephrine, collagen, (thrombin)

o Normal response to ristocetin

Bernard-Soulier syndromeo Platelet aggregation test

o Failure to aggregate in the presence of ristocetino Aggregation by other agonists (ADP, collagen,

epinephrine): normalo Response to low-dose thrombin: may be delayed

o Dense (δ) granule defects ~ storage pool deficiency

o α granule defects ~ gray platelet syndromeo Heterogeneous group of disorders

o Mild to moderate bleeding diathesiso Abnormalities in platelet aggregation

Platelet storage granule defects

Precautions Prior To Studying Platelet Aggregation Aspirin-containing compounds should be excluded for at

least 10 days prior to testing, as Aspirin interferes with the release reaction.

Ingestion of other drugs known to influence platelet function should also be avoided for at least the time required for their elimination from the circulation. These include certain antihistamines, antibiotics, and anti-depressants. A check should be made of any drugs being prescribed before performing platelet function testing.

Chylomicrons can interfere with the measurement of platelet aggregation, studies should not be carried out shortly after a fatty meal.

Many other “normal” dietary constituents, including alcohol, onions, garlic, peppers, and ginger, may also inhibit platelet aggregation.

Chilling activates platelets, and so the blood is processed at 20°C –25°C.

Comment In evaluating patients with suspected platelet

disorders, the aggregating agents most commonly used are ADP in varying concentrations, collagen, epinephrine, and ristocetin.

Aspirin, aspirin compounds, and anti-inflammatory drugs inhibit the secondary wave of aggregation by inhibiting the release reaction of the platelet.

The intensity of platelet aggregation may be estimated by recording the change in absorbance as a percentage of the difference in absorbance between platelet-rich and platelet-poor plasma.

This has limited usefulness because absorbance is dependent on the size and density of platelet clumping and the number of platelets that aggregate.

Drugs and PLT FunctionDrugs and PLT Function Aspirin:

Acetylsalicyclic acid Irreversibly inhibits Cyclooxygenase

Clopidogrel : Plavix Irreversibly inhibits P2Y12

Dipyridamole: inhibits Thromboxane synthase

Abciximab ReoPro inhibits GP IIb/IIIa