Clinical features:
(1) negative past history for von Willebrand disease
(2) presence of essential thrombocythemia
(3) development of a spontaneous bleeding disorder
(4) presence of a very high platelet count (over 1,000,000 per µL or more)
(5) prolonged bleeding time
(6) normal Factor VIII coagulant activity
(7) normal von Willebrand antigen concentration
(8) low von Willebrand factor Ristocetin cofactor activity
(9) low collagen binding activity
(10) absence of intermediate and large von Willebrand factor multimers, similar to Type 2B vWD
(11) correction of bleeding disorder and laboratory test abnormalities with normalization of platelet count
Differential diagnosis:
(1) ET arising in a patient with hereditary vWD
(2) excessive dose of aspirin
(3) other coagulopathy in a patient with ET
