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BACKGROUND Considering the physiological changes in serum procalcitonin (PCT) levels in newborns due to age, we recently established an age-specific percentile-based reference curve for serum PCT level. The present study aimed to determine the best cutoff percentile line using this reference curve for the differentiation between infected and colonized preterm infants. METHODS A total of 52 preterm infants with positive bacterial culture (9 with bacterial infection, 43 with colonization) were enrolled within the study period. The 97.5th, 95.0th, 92.5th, 90.0th, 80.0th, 70.0th, 60.0th, and 50.0th percentile lines were drawn in the reference curve. PCT levels in infected or colonized infants were used, and sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were calculated. The best cutoff percentile line was determined in the receiver operating characteristic curve analysis. RESULTS Of the 52 preterm infants, 9 were infected (5 and 4 infants with an onset of < 7 days and ≥ 7 days after birth, respectively), whereas 43 were colonized (6 and 37 infants with an onset of < 7 days and ≥ 7 days after birth, respectively). The best cutoff percentile lines were the 90.0th percentile (sensitivity, 0.800; specificity, 0.833; PPV, 0.800; NPV, 0.833) and 97.5th percentile (sensitivity, 1.00; specificity, 0.973; PPV, 0.800; NPV, 1.00) in infants with an onset of < 7 days and ≥ 7 days after birth, respectively. CONCLUSIONS The age-specific percentile-based reference curve for serum PCT level is clinically applicable as a new tool for diagnosing infections in preterm infants with positive culture results, particularly at ≥ 7 days after birth.