Clinical definitions of acute kidney injury (AKI) have been based on an increase in serum creatinine and a decrease in urine output. However, applying this definition to neonates remains challenging because of the normal renal physiologic features that serum creatinine levels are expected to increase in the first days after birth, and impaired sodium reabsorption and concentrating ability.

Because of several limitations of early detection of AKI, investigators are focused on identifying biomarkers that predict AKI before an increase serum creatinine level.

Investigators will collect urine from preterm infants before and after administrating ibuprofen for closing patent ductus arteriosus. To identify novel biomarkers, investigators will analyze urine by proteomics. To verify those biomarkers, investigators will use initial urine on the first day of life from preterm infants who diagnosed AKI within 7 days after birth without any risk factors for AKI and enrolled institutional bio-repository.

Inclusion Criteria:

• Preterm infants less than 32 weeks gestational age or birth weight less than 1,500 g admitted to the neonatal intensive care unit at Seoul National University Children's hospital

Exclusion Criteria:

• Congenital heart disease
• Known major congenital anomalies of the kidney and urinary tract
• Other genetic syndromes or medical conditions that preclude enrollment per judgement of the attending neonatologist

1. Preterm infants with gestational age<32 weeks or birth weight<1,500 g who enrolled our institutional prospective cohort study for biorepository.
2. Preterm infants with gestational age<32 weeks or birth weight< 1,500 g who need ibuprofen treatment for closing patent ductus arteriosus.