Platelet aggregation studies

Principle

• Light transmission aggregometry (LTA, Born method): platelet-rich plasma (PRP) is stirred at 37 °C in a cuvette. When an agonist is added, platelets aggregate and the plasma becomes clearer, increasing light transmission.
• Platelet-poor plasma (PPP) from the same patient sets 100% transmission; PRP sets 0%.

Specimen

• 3.2% sodium citrate, 9:1, collected with minimal stasis; discard the first few mL.
• Keep at room temperature (cold activates platelets); rest about 30 min; complete testing within about 4 h.
• PRP: centrifuge at about 150–200 g for 10–15 min at room temperature. PPP: high-speed centrifugation of the remaining sample.
• Avoid aspirin, NSAIDs, clopidogrel and other antiplatelet drugs for about 7–10 days before testing when clinically possible.

Reagents and equipment

• Agonists: ADP, collagen, adrenaline (epinephrine), arachidonic acid, ristocetin (high dose about 1.2–1.5 mg/mL, low dose about 0.5–0.6 mg/mL), thrombin receptor activating peptide, calcium ionophore.
• Aggregometer with stirring cuvettes.

Interpretation

• Glanzmann thrombasthenia (GPIIb/IIIa defect): absent aggregation to all agonists except ristocetin, which gives agglutination.
• Bernard–Soulier syndrome (GPIb-IX-V defect): absent ristocetin response, normal with other agonists; giant platelets and thrombocytopenia.
• von Willebrand disease: reduced ristocetin response that corrects when normal plasma is added.
• Type 2B VWD and platelet-type VWD: enhanced aggregation to low-dose ristocetin.
• Aspirin effect or cyclooxygenase defect: absent arachidonic acid response, loss of secondary wave with ADP and adrenaline, reduced collagen response.
• Storage pool or release defects: primary wave only with ADP and adrenaline, reduced collagen.
• Clopidogrel/P2Y12 defect: reduced ADP response.

Quality control and pitfalls

• Thrombocytopenia (PRP count below about 100 × 10^9/L), lipaemia, haemolysis and clots impair results.
• Test a normal control subject with each run where possible.

Clinical use

• Diagnosis of inherited and acquired platelet function disorders; assessment of antiplatelet drug effect.