Introduction. Primary thrombocytosis is very rare in children; reactive thrombocytosis is frequently observed in children with infections, anemia, and many other causes. Aims and Objectives. To identify the etiology of thrombocytosis in children and to analyze platelet indices (MPV, PDW, and PCT) in children with thrombocytosis. Study Design. A prospective observational study. Material and Methods. A total of 1000 patients with thrombocytosis (platelet > ) were studied over a period of 2 years. Platelet distribution width (PDW), mean platelet volume (MPV), and plateletcrit (PCT) were noted. Results. Of 1000 patients, 99.8% had secondary thrombocytosis and only two children had primary thrombocytosis (chronic myeloid leukemia and acute myelogenous leukemia, M7). The majority of the children belonged to the age group of 1month to 2 years (39.7%) and male to female ratio was 1.6？:？1. Infection with anemia (48.3%) was the most common cause of secondary thrombocytosis followed by iron deficiency alone (17.2%) and infection alone (16.2%). Respiratory infection (28.3%) was the predominant infectious cause observed. Thrombocytosis was commonly associated with IDA among all causes of anemia and severity of thrombocytosis increased with severity of anemia ( ). With increasing platelet count, there was a decrease in MPV (<0.001). Platelet count and mean PDW among children with infection and anemia were significantly higher than those among children with infection alone and anemia alone. None were observed to have thromboembolic manifestations. Conclusions. Primary thrombocytosis is extremely rare in children than secondary thrombocytosis. Infections in association with anemia are most commonly associated with reactive thrombocytosis and severity of thrombocytosis increases with severity of anemia. 1. Introduction Thrombocytosis refers to platelet count >4,00,000/μL in the peripheral blood [1]. With the widespread use of electronic cell counters and the subsequent availability of a platelet count as part of a routine blood count, thrombocytosis is more often observed as an unexpected finding. Thus, an elevated platelet count has become an important clinical problem for differential diagnosis of various pathological and physiological processes [2, 3]. Thrombocytosis is classified according to its origin into primary and secondary forms. Primary (clonal) thrombocytosis is a myeloproliferative disorder, caused by abnormal and uncontrolled expansion of haematopoietic cells, which is likely to be complicated by thromboembolism [2]. Secondary (or reactive) thrombocytosis
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