Thrombocytosis in neonates and young infants: a report of 25 patients with platelet counts of ⩾1 000 000 μl −1

Objective: Thrombocytosis has been reported in neonates and young infants, but little is known of its prevalence, timing of onset, associated conditions, sequelae and outcomes. To better understand this condition, we used the data repositories of a multi-hospital health-care system to identify all individuals ⩽140 days old (20 weeks) who, during the past 6 years, had a platelet count of ⩾1 000 000 μl −1 . Study Design: We identified all infants with extreme thrombocytosis (using the Sutor definition of a platelet count of ⩾1 000 000 μl −1 ) during the period of January 2003 through December 2008 in any Intermountain Healthcare facility. We obtained the information provided in this report from electronic and paper records. Result: Among 40 471 infants who had one or more platelet counts performed in this period, 25 had extreme thrombocytosis. No cases were identified in the first week after birth, 40% were recognized between the second and fourth weeks and 40% between the fifth and eighth week. The prevalence of thrombocytosis had no relationship with birth weight or gestational age but a slight predominance of female patients (15/25) was noted. In all, 26 episodes were found among the 25 infants: 12 episodes involved an antecedent infectious disease, 8 had an antecedent surgical procedure, 4 had the anemia of prematurity and 1 each had congenital adrenal hyperplasia and opiate withdrawal syndrome. No pathological thromboses or hemorrhages or other sequelae were detected and all episodes resolved with no deaths. Conclusion: The thrombocytosis cases that we report were all consistent with reactive thrombocytosis (also known as secondary thrombocytosis); none seemed to be essential (primary) thrombocytosis. We speculate that the pathogenesis involves increased platelet production due to megakaryopoietic stimulators induced by an infectious or inflammatory condition. From this series and previous reports, young infants with platelet counts up to 1 300 000 μl −1 do not seem to have a significant risk of thrombotic or hemorrhagic problems, and do not generally require anti-platelet or cytoreductive treatment.