Extreme Hyperleukocytosis in an Extremely Preterm Infant

An extremely preterm male infant at 25.1 weeks gestation developed extreme hyperleukocytosis on Day of life (DOL) 3, with a peak white blood cell (WBC) count of 146.5 ×109/L on DOL 5. The patient was admitted to the NICU due to prematurity, respiratory distress, and suspected sepsis. Laboratory analyses were done to exclude sepsis with a leukemoid-like reaction, Down syndrome by karyotype, leukemia, or other myeloproliferative disorders. After a meticulous investigation and ruling out the most common causes of leukocytosis in preterm neonates, we concluded that the hyperleukocytosis in this extreme premature infant was likely caused by maternal chorioamnionitis and fetal funisitis as reported by placental pathology. We present this rare case of hyperleukocytosis in an extremely premature infant to highlight the importance of a thorough investigation, including sources of infection in the mother's history and placental pathology report, to determine the cause of hyperleukocytosis in the preterm neonates.