Late-onset neutropenia: defining limits of neutrophil count in very low birth weight infants

Objective:to evaluate the incidence, onset, duration, characteristics and importance of late-onset neutropenia (defined as absolute neutrophil count<1500 μl−1 at 3 weeks of age or later) in a group of very low birth weight (vlbw) infants.study design:routine complete blood cell counts (cbcs) obtained from vlbw infants over a period of 7 years were gathered retrospectively, including those of newborns with weekly cbcs taken over a duration of at least 3 weeks. data were obtained from between january 2003 and december 2009.result:cbcs of 399 newborns were included. values were obtained from birth to 36 weeks of postnatal age. late-onset neutropenia was observed in 259 cases (65%). neutropenic infants had a mean of 0.5 weeks lower gestational age. late-onset neutropenia was more frequent in children with intraventricular hemorrhage but not in patients who received erythropoietin. the median age of neutropenia onset was 7 weeks in extremely low birth weight infants and 6 weeks in vlbw infants. the fifth percentile of neutrophils between weeks 3 and 4 was 1280 μl−1 and between weeks 13 and 15 was 500 μl−1. the average duration was 2 weeks with normalized values after 18 weeks.conclusion:a neutrophil count <1500 μl−1 after the third week of life is frequently observed in vlbw infants and should not be used as a lower reference limit. the fifth percentile varies according to postnatal age from around 1300 μl−1 in week 4 of life, decreasing to a nadir of 500 μl−1 between 3 and 4 months of age. values normalize in the first year of life.