(See also Overview of Myeloproliferative Disorders.)

Reactive thrombocytosis is an elevated platelet count (> 450,000/μL) that develops secondary to another disorder.

Some causes of reactive thrombocytosis include

There are also congenital familial thrombocytoses such as those due to thrombopoietin and thrombopoietin receptor gene mutations. For thrombocytosis that is not secondary to another disorder, see Essential Thrombocythemia.

Platelet function is usually normal. Unlike in essential thrombocythemia, reactive thrombocytosis does not increase the risk of thrombotic or hemorrhagic complications unless patients have severe arterial disease or prolonged immobility.

With secondary thrombocytosis, the platelet count is usually <1,000,000/μL, and the cause may be obvious from the history and physical examination (perhaps with confirmatory testing). CBC and peripheral blood smear should help suggest iron deficiency or hemolysis.

If a cause of secondary thrombocythemia is not obvious, patients should be evaluated for a myeloproliferative disorder. Such evaluation may include cytogenetic studies, including Philadelphia chromosome or BCR-ABL assay, and possibly bone marrow examination, especially in patients with anemia, macrocytosis, leukopenia, and/or hepatosplenomegaly.

Treatment of the underlying disorder usually returns the platelet count to normal.