The aims of the present study were to 1) establish age-specific percentile-based reference curves of serum PCT in late preterm and preterm infants early after birth and determine the physiological features of PCT; 2) determine the age when infants’ serum PCT reaches ≤0.1 ng/mL, which is the reference range for adults and children, in preterm infants; and 3) evaluate changes in serum PCT concentrations in preterm infants who develop late-onset sepsis or bacterial infection using the 95-percentile reference curve we established. This evidence concerns the gene CALCA and bacterial infectious disease.