Background: Sampling of salivary cortisol is non-invasive and important for the evaluation of the hypothalamic–pituitary–adrenal axis function and stress levels. However, the reference values for salivary cortisol measured by liquid chromatography–tandem mass spectrometry (LC-MS/MS) in healthy infants are unclear. The aim of this study was to establish the reference values for salivary cortisol levels in healthy infants. Methods: This study was a prospective observational cohort study following the participants until the age of 6 months. We analyzed 71 healthy, full-term infants at age 1 month between December 2017 and March 2018. We repeated saliva sampling every month, measured the salivary cortisol levels in the early morning by LC-MS/MS, and took the subjects’ medical history by questionnaire. Results: The minimum, 25th, 50th, 75th percentile, and maximum salivary cortisol levels were 0.08, 1.11, 2.21, 5.18, and 30.35 nmol/L, respectively. Conclusions: We established the reference values for salivary cortisol in young infants using LC-MS/MS.
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