NOTE: The risk factors are the same as for polycythemia vera.
Parameters:
(1) age
(2) history of thrombosis
(3) cardiovascular risk factors (smoking, diabetes mellitus, hypertension, hyperlipidemia)
(4) platelet count
Criteria for low risk - all of the following:
(1) age < 60 years
(2) no history of thrombosis
(3) platelet count < 1,500,000 per µL
(4) no cardiovascular risk factors
Criteria for high risk - one or more of the following:
(1) age >= 60 years
(2) history of thrombosis
Criteria for indeterminate risk - neither low or high risk, or all of the following:
(1) age < 60 years
(2) either platelet count >= 1,500,000 per µL OR cardiovascular risk factors present
(3) no history of thrombosis
