Age-dependent changes in the incidence and etiology of childhood thrombocytosis.

To determine the incidence and etiology of childhood thrombocytosis, over 15,000 platelet counts in 7,539 patients performed at a single regional hospital were reviewed. When thrombocytosis was defined as > or =500 x 10(9)/l of platelet counts, the condition could be diagnosed in 6.0% (456 cases) of the patients. All patients were classified as having secondary thrombocytosis. The incidence of thrombocytosis dramatically changed throughout child development; it was 12.5% in neonates, peaked to 35.8% in 1-month-old infants and then returned to 12.9% in 6- to 11-month-old infants. Thereafter, it gradually decreased with age to only 0.6% in 11- to 15-year-old children. Frequent causes of thrombocytosis included infection (67.5%), Kawasaki disease (9.4%), prematurity (7.7%) and iron deficiency anemia (6.4%). Thrombocytosis was an incidental finding in a substantial population of early infants. Thrombocytosis as a reaction to several types of infection and Kawasaki disease was more common in children under 7 years old, while autoimmune disease and tissue damage were major causes in children aged 11-15 years. No child had thromboembolic complications. These findings indicate that childhood thrombocytosis is a benign condition and its incidence and etiology seem to depend on age.