With the exception of usual nursing care, other therapeutic treatments that may have induced preterm infant’s acute pain (e.g., IV, puncture, intubation) and affected the
concentration of preterm infants’ stress hormones were not administered to the preterm infants in this study for a minimum of 12 h before or after the initiation of urine collection. This was done under the permission of the medical doctors in charge. If such therapeutic treatment was needed during that period, the subject was withdrawn from the study