General Approach in Investigation 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

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 + ]light

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

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

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

PRP Aggregometry Agonist & Patterns

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 .

Interpretation 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“

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 of 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).

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 deficiencyo 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.

Lipemia 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.