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Background: Procalcitonin (PCT) may be a more sensitive marker for neonatal bacterial infections than C-reactive protein (CRP). However, the reference intervals of serum PCT were not sufficiently studied in neonates older than 1 week of age, especially for very low birth weight infants. Objectives: This study investigated the reference level of serum PCT for neonates according to gestational age (GA) and postnatal age (PNA). Methods: Serum PCT was measured in 914 blood samples from 7-60 days after birth in 415 neonates including 184 premature infants. Infants with sepsis, congenital anomaly, or clinically evident intra-amniotic infections were excluded. Multivariate analysis of covariance was used to detect the interaction between GA and PNA. To compare subgroups dichotomized by GA and PNA, analysis of covariance was performed with clinical parameters as covariates to obtain an adjusted p value. Results: Serum PCT levels were negatively correlated with GA, PNA, birth weight, birth height, and platelet count, and positively correlated with white blood cell count, absolute neutrophil count, hematocrit, and serum CRP after logarithmic transformation. Reference intervals of serum PCT were established according to GA and PNA. High PCT levels were found in infants with GA ≤32 weeks and PNA 7-30 days. Conclusion: The reference levels of serum PCT were determined according to GA and PNA. As the reference PCT levels of infants with GA ≤32 weeks were affected by PNA, cautious interpretation of PCT levels in these infants is warranted. © 2015 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]