Asymptomatic late thrombocytosis is a common finding in very preterm infants even in the absence of erythropoietin treatment.

Objectives: Thrombocytosis is more prevalent in pediatric than in adult patients and is associated with complications or worsened outcomes after vascular events. This study aimed to determine the prevalence of thrombocytosis in very preterm infants who had not received human recombinant erythropoietin treatment (rHuEPO) and its relationship with other hematological parameters and clinical complications.

Methods: We performed a retrospective study of hematological and clinical data of very preterm infants who were admitted to our unit in their first 48 hours of life and stayed for longer than 1 week.

Results: Thrombocytosis was prevalent (32.6% of patients) in very preterm infants (≤32 weeks of gestational age, n = 193) who had not received rHuEPO. The platelet count was positively correlated with calendar age. Infants with thrombocytosis were significantly more premature (28.0 ± 2.1 versus 29.6 ± 2.2 weeks) and had a lower birth weight (1036 ± 304 versus 1303 ± 304) than those without thrombocytosis. Thrombocytosis was associated with retinopathy of prematurity after adjusting for gestational age and comorbidities, but not with other prematurity-associated complications.

Conclusions: Late asymptomatic thrombocytosis is common in very preterm infants at approximately 1 month of postnatal age and it may be associated with retinopathy of prematurity.