Platelet Function + Thrombocytopenia

Question 1

Name platelet agonists?

Which are strong, which are weak?

Answer 1

Strong:
Collagen
Arachadonic acid
Thrombin-related activation peptide.
Thromboxane A2 mimetic U46619.

Weak:
ADP
Epinephrine

Induces agglutination, not aggregation:
Ristocetin

Question 2

What is the pattern of platelet aggregation with:
A) Bernard Soulier Syndrome
B) Glanzmann Thrombasthenia
C) VWD - 2B
D) Aspirin effect
E) Clopidogrel Effect

Answer 2

A) Absent or markedly reduced platelet agglutination with Ristocetin. VWD gives similiar results. MACROTHROMBOCYTOPENIA can differentiate.
B) Absent or markedly impaired aggregation to all agonists except Ristocetin. Ristocetin-induced agglutination shows only primary wave - aggregation cannot occur because fibrinogen cannot bind.
Afibrinogenaemia gives similar results.
C) Agglutination with low dose Ristocetin e.g. 0.5 mg/mL.
D) Absent aggregation to Arachidonic acid. Primary wave aggregation only with ADP. Decreased or absent aggregation with collagen.
E) Absent aggregation with ADP

Question 3

Potential side effects of DDAVP used in bleeding disorders?

Answer 3

During infusion:
Flushing, hypotension, Headache, tachycardia; must be given slowly

Post-infusion:
Hyponatremia and increased seizure risk due to same

Tachyphylaxis; can only use ~2-3 doses (~48-72hrs)

Question 4

Tranexamic Acid; mechanism and adverse effects/contraindications

Answer 4

1. Lysine analogue which inhibits plasmin, leading to clot stabilization
2. Slight increased thrombogenic risk, lowers seizure threshold, cannot be used in active VTE, active DIC manifesting with thrombosis, or gross hematuria