1. General Approach in Investigation of Haemostasis
Platelets aggregation

2. Studying Platelets Disorders
Numbers
CBC
PLT count
PLT morphology
Function
Bleeding Time (BT)
Platelet Aggregation
Whole blood aggregation
Platelet-rich plasma aggregation

3. Platelets contribute to Hemostasis in two main ways:
Primary haemostatic plug :
Adhesion
Aggregation
Secretion
Secondary Haemostatic plug:
Procoagulant activities are generated

4. Platelets granules
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.

5. Why Platelet aggregation testing
Evaluation of suspected hereditary and acquired disorders of platelet function.
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.
NSAIDs: Nonsteroidal anti-inflammatory drugs, like Ibubrufin, asprin, Naproxen are drugs with analgesic (also known as a painkiller) and antipyretic (fever-reducing) effects and which have, in higher doses, anti-inflammatory effects.
Clopidogrel is an oral, thienopyridine class antiplatelet agent used to inhibit blood clots in coronary artery disease, peripheral vascular disease, and cerebrovascular disease.
Uremia is a term used to loosely describe the illness accompanying kidney failure .

6. Principle
Using Aggregating agents to induce platelet aggregation or cause platelets to release endogenous ADP, ATP or both.
Platelet aggregation is studied by means of a platelet aggregometer, Used Principle:
Photo-optical Method
luminescence technology (Platelet Lumiaggregometry)
Electrical Impedance Method

7. AGGREGATING AGENTS

8. 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.

9. 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.
Firefly luciferin is the luciferin, or light-emitting compound, found in many firefly (Lampyridae) species, a yellow light emission from. As with all other luciferins, oxygen is required to elicit light; however, it has also been found that ATP and magnesium are required for light emission
[A] + [B] → [AB◊] → [Products] + light

10. 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.
The optical density of platelet-rich plasma falls as platelets form aggregates. The amount, and to some extent the rate, of fall is largely dependent on platelet reactivity, provided that all other variables (e.g. platelet count, mixing speed, and temperature) are controlled. The optical density changes are monitored, wherein the optical density of platelet-rich plasma reflects the degree of platelet aggregation induced by one of a variety of agonists. The optical density is monitored using an aggregometer connected to a chart recorder so results may be recorded graphically.

12. 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.

13. Patient Sample – 3.2% citrated WB
Approximately 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
Issues
Sample quality is critical
Fibrinogen levels are important
Agonists must be prepared fresh daily
Thrombocytopenia makes result interpretation difficult
Complete patient history is essential

14. Sample Quality for PRP PLT Aggregometry
Clean venipunture
Hemolysis = increased plasma ADP = prematurely activated platelets
Lipemia may obscure OD during PLT aggregation
PRP contact with glass = prematurely activated PLTs
Keep samples capped to prevent loss of CO2 – change sample pH ( PRP PH should be maintained within the range 7.7–8.0)
Store room temp – prevent inhibition of PLT aggregation
Perform testing within 3 hours of sample collection

15. PRP Aggregometry Agonist & Patterns
ADP (at appropriate concentration)
Biphasic curve: 1o and 2o waves
Note: if ADP is added at too low or too high concentration, it will not get biphasic response
Epinephrine
Biphasic curve
Collagen
Lag phase followed by 2o wave only

16. Thrombin Arachidonic acid Serotonin A biphasic however Ristocetin
Biphasic curve. Irreversible aggregation only (does not require cyclooxygenase)
Arachidonic acid
2o wave only; assesses cyclooxygenase pathway
Serotonin
A primary wave of aggregation with a maximum of 10% to 30% transmittance followed by disaggregation.
Ristocetin is an antibiotic, It causes platelet agglutination and blood coagulation and is used to assay those functions in vitro, e.g., to diagnose von Willebrand disease (vWD) or the Bernard-Soulier syndrome. Platelet agglutination caused by ristocetin can occur only in the presence of large multimers of von Willebrand factor, so if ristocetin is added to blood lacking the factor , it will not coagulate.

17. 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.

18. 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.

19. 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
Bernard-Soulier syndrome,
Platelet storage pool defects
Idiopathic thrombocytopenia purpura (ITP).

20. Bernard-Soulier syndrome
Platelet aggregation test
Failure to aggregate in the presence of ristocetin
Aggregation by other agonists (ADP, collagen, epinephrine): normal
Response to low-dose thrombin: may be delayed
Platelet storage granule defects
Dense (δ) granule defects ~ storage pool deficiency
α granule defects ~ gray platelet syndrome
Heterogeneous group of disorders
Mild to moderate bleeding diathesis
Abnormalities in platelet aggregation
bleeding diathesis (bleeding tendency) : is an unusual susceptibility to bleeding (hemorrhage) mostly due to hypocoagulability, in turn caused by a coagulopathy

21. 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.