An elevated platelet count can occur in patients with medical or surgical conditions as a reactive phenomenon which is usually cytokine-driven. In other patients, thrombocytosis is a marker of an underlying clonal bone marrow disorder such as a myeloproliferative neoplasm or a myelodysplastic syndrome. A high platelet count has the potential to be associated with vasomotor symptoms, thrombosis and bleeding, and these risks and their management also require consideration.
Initial evaluation of a high platelet count should exclude spurious causes of elevation, followed by systematic consideration of known causes of reactive thrombocytosis which may be aided by assessment of inflammatory markers and iron status. Classification of a clonal marrow cause of a high platelet count will involve assessment of blood film morphology, molecular markers and, in most settings, bone marrow findings. The expansion of our knowledge about genetic abnormalities occurring in primary bone marrow disorders causing thrombocytosis has improved our ability to differentiate these conditions from reactive causes, so that patients can be appropriately monitored and managed.