Complete blood count reference intervals for extremely preterm neonates

Preterm infants exhibit dynamic hematological profiles during the first postnatal days, necessitating robust reference intervals for clinical interpretation. The aim of this study is to establish comprehensive reference intervals for complete blood counts in preterm neonates during the first five postnatal days, stratified by gestational age and postnatal age. Data from preterm neonates (23 + 0-34 + 0 weeks gestation) admitted to the neonatal intensive care unit Graz (2008-2023) were analyzed. Percentiles (2.5th, 10th, 50th, 90th, 97.5th) were generated for the following hematological parameters included in the full/complete and differential blood count (FBC, CBC): leukocytes, thrombocytes, hemoglobin, hematocrit, neutrophilic granulocytes, and lymphocytes. Stringent exclusion criteria minimized confounding factors. Comparisons were made between appropriate-for-gestational age (AGA) and small-for-gestational age (SGA) neonates. A total of 3128 eligible preterm neonates with a median (minimum; maximum) gestational age of 32.1 (23.1; 34.0) weeks and a birth weight of 1700 (297; 3210) grams were included for analysis. Reference intervals were established for each parameter across gestational ages and postnatal days. Significant differences were observed between AGA and SGA neonates, particularly for leukocytes, thrombocytes, and neutrophilic granulocyte with lower values in SGA infants. Hemoglobin concentration and hematocrit were largely similar between groups, with some exceptions at specific gestational ages.Conclusion: This study provides reference intervals for hematological parameters in preterm neonates, enabling improved clinical interpretation and identification of deviations. The large sample size and strict exclusion criteria enhance the validity of these reference intervals. The observed differences between AGA and SGA neonates warrant further investigation to understand the clinical implications of fetal growth restriction on neonatal hemato-/lymphopoiesis.