RESUMO
To identify the etiology of thrombocytosis in various age groups and to evaluate the effectiveness of platelet indices in differentiating reactive and clonal thrombocytosis, an observational, prospective review of patients with platelet count of 600 x 10(9)/L or more performed by using coulter counter STKS (Coulter Electronic, Kerfeld, Germany). Extreme thrombocytosis defined as platelet count of 1000 x 10(9)/L or more. Of 1068 patients, 91.8% had reactive and 8.2% had clonal thrombocytosis. Frequent causes of reactive thrombocytosis were infections (44.9%), tissue injury (11.4%) and rebound thrombocytosis (10.2%). Fifty-five patients had extreme thrombocytosis, main aetiologies were secondary and clonal thrombocytosis that seen in 72.7% and 27.3% of cases respectively. Comparison of platelet indices showed; that cases with reactive thrombocytosis had low mean platelet volume and platelet distribution width. We concluded that thrombocytosis could be a response to various physiological and pathological processes. Low MPV and PDW in patients with high platelet counts strongly suggest reactive etiology.