Coated-Platelet Levels May Explain Some Variability in Clinical Phenotype of Severe Hemophilia.

Objective: To determine if variability in clinical phenotype of severe hemophilia patients is influenced by the percentage of coated-platelets.

Background: Co-activation of platelets with thrombin and collagen results in a unique subset of platelets, with high levels of alpha granule proteins on their surface, such as Factor V, fibrinogen, von Willebrand factor, and thrombospondin. This subset of activated platelets is referred to as coated-platelets (

Methods: After informed consent, 3–5 ml of blood was drawn from patients with severe hemophilia (Factor VIII <1%) and healthy controls. The number of bleeding episodes reported in the last 6 months was taken as an indicator of clinical phenotype.

Results: In 6 patients with more than 3 bleeds reported in the last 6 months (14 ± 4.9 bleeds; mean ± 1SD), the average coated-platelets percentage was 22.3 ± 11.2%. In 17 patients with 3 or less bleeds (1.6 ± 1.3) in the last 6 months, the average coated-platelets percentage was 37.6 ± 12.0%, a difference that was statistically significant (p=0.012). In healthy controls (n=12), the mean coated-platelets were 30.2 ± 9.5%.

Conclusion: A higher percentage of coated-platelets may provide a better procoagulant locus for residual Factor VIII, thereby reducing the number of clinical bleeding episodes and partially explaining some variability observed in clinical phenotype of severe hemophilia.