2.3.1. Stress hormones
The concentration of urine stress hormone was measured twice (i.e., before and after the intervention) to examine the effect of the intervention on the infants’ stress level. The
urine was collected over a 24-hour period using a urine collector from the perineal area of the preterm infants. The concentration of cortisol was measured by Radio Immuno
Assay using the following criteria: detection limit 5 mg/dl; intra-assay CV 3–5%; inter-assay CV 4–6%. The concentration of norepinephrine was measured by high performance liquid chromatography using the following criteria: detection limit 5 mg/dl; intra-assay CV 6–8%; inter-assay CV 7–10%. The increase in cortisol or norepinephrine levels could be interpreted as a sign of stress (