and empiric antiplatelet therapy is not routinely recommended even for extreme thrombocytosis Clinical Considerations Clinical Recognition Thrombocytosis is defined as a platelet count greater than 450 × 103/μL Primary thrombocytosis is rare in the pediatric population Hereditary or familial forms have been associated with gene variants that encode thrombopoietin and the thrombopoietin receptors, and transmit in an autosomal dominant fashion Clonal forms of thrombocytosis result from myeloproliferative syndromes; these are extremely rare in children The pathophysiology and natural history of primary thrombocytosis have been predominantly explored in the adult population Evidence of increased risk of thrombotic and hemorrhagic events has been reported in children but to a lesser degree than observed in adults Risk assessment and stratification tools have not been validated in children The degree of thrombocytosis can range from mild (>450 × 103/μL) to extreme (>1,500 × 103/μL) regardless of a primary or secondary etiology Secondary thrombocytosis is common and occurs in the setting of infection, inflammation, or other physiologic stress ( Table 93.11 ) Clinical Assessment/Diagnostic Testing Thrombocytosis is usually an incidental finding Spurious causes of thrombocytosis such as cell fragments or microorganisms should be excluded by reviewing a peripheral blood smear Initial evaluation should focus on identifying an inciting cause In the absence of symptoms, laboratory evidence of acute inflammation such as C-reactive protein, von Willebrand factor (VWF), fibrinogen, and proinflammatory cytokines (IL6) may suggest an underlying inflammatory state The presence of Howell–Jolly bodies on peripheral blood film may suggest an asplenic or functionally asplenic state