Thrombocytosis in childhood.

Until recently, thrombocytoses in childhood were considered to be rare. The literature on this subject is scarce and contradictory. When thrombocytosis is defined as a platelet count of more than 500 x 10(9)/L (500 000/mm3 or 500 000/microL), the occurrence in routine examinations at pediatric hospitals can be estimated as 3 to 13%. These are almost exclusively secondary thrombocytoses, which are the result of infections (most frequently), trauma and surgery, hypoxemia, immunologic disturbances, premature birth, gastrointestinal disorders, medications, stress situations, or previous loss of platelets. The secondary thrombocytosis usually has less then 800 x 10(9)/L platelets, is temporary, and occurs predominantly in infants and young children. If only thrombocytosis is present, thrombotic complications are practically nonexistent. Antithrombotic prophylaxis is not necessary unless other risk factors for thrombosis are present, such as vessel damage, permanent iron deficiency, hyperviscosity, immobilization. Primary thrombocytoses due to a defect of stem cells are extremely infrequent in childhood.