Reference intervals for six salivary cortisol measures based on the Croatian Late Adolescence Stress Study (CLASS)

INTRODUCTION: The aim of this nested study is to provide the reference intervals for already published measurements of salivary cortisol from the Croatian Adolescence Stress Study (CLASS). MATERIAL AND METHODS: A total of 969 individuals (372 males and 597 females) were included in the reference sample (age range: 18-21 years). Salivary cortisol concentrations were determined by the enzyme immunoassay (LUCIO-Medical ELISA Salivary Cortisol Kit, Nal von Minden, Germany) in the Department of Medical Laboratory Diagnostics, University Hospital Split. Nonparametric statistics were used for calculating the reference intervals (RIs) and 90% confidence intervals (90% CIs). RESULTS: The lower limits of RIs determined by the direct method were higher in females (> 10%) than in males for the cortisol concentrations at awakening (SCC(0)), 30 to 45 after awakening (SCC(30-45)) and at bedtime (SCC(bedtime)). The upper limits of RIs for the SCC(bedtime) were higher (> 10%) in males than in females. Females also had higher upper limits of RIs for the cortisol awakening response (CAR) and the diurnal cortisol slope (DCS) and higher lower limits of RIs for the CAR and the area under the curve with respect to ground (AUC(G)). The lower limits of RIs for the DCS were higher in males than in females. CONCLUSIONS: Obtained reference values open the arena for introducing salivary bioscience in Croatian clinical laboratory practice and provide important data for better understanding of gender differences in adaptation to stress during late adolescence.