Influence of age, gender and body mass index on late-night salivary cortisol in healthy adults

Background: Late-night salivary cortisol (LNSC) is one of the most reliable tests to screen for endogenous Cushing syndrome. This test is simple, inexpensive and noninvasive and has high sensitivity and specificity. The aim of our study was to analyze the putative influence of age, gender and body mass index (BMI) on LNSC levels in a healthy population. Methods: Cross-sectional study conducted in healthy adults. Midnight saliva samples were collected at home. Participants refrained from teeth brushing, eating or drinking for 2 h prior to collection. Salivary cortisol measured by electrochemiluminescence immunoassay (ECLIA). The study was approved by the Ethics Committee of the hospital (number 140073). Results: We evaluated 122 nonsmoking healthy volunteers. Mean age was 35 +/- 14 years (range, 18-74 years); 63% were women. Mean BMI was 24 +/- 3 kg/m(2), blood pressure 115/74 mmHg and fasting plasma glucose 4.8 +/- 0.5 mmol/L. LNSC presented a non-Gaussian distribution; the median was 3.58 (range, 0.55-8.55) nmol/L (0.13 [range, 0.02-0.31] mu g/dL), and the 97.5th percentile (P97.5) was 8.3 nmol/L (0.3 mu g/dL). Multiple linear regression disclosed a significant positive association between salivary cortisol levels and age (r(2) = 0.21, p < 0.001), but no association with gender (p = 0.105) or BMI (p = 0.119). Accordingly, participants aged >50 years had significantly higher salivary cortisol as compared to those aged <50 years (5.24 nmol/L [0.19 mu g/dL] vs. 3.31 nmol/L [0.12 mu g/dL], respectively, p < 0.001). Conclusions: The maximum reference value (P97.5) of LNSC was set at 8.3 nmol/L (0.3 mu g/dL) using ECLIA. Advanced age was associated with higher LNSC levels, with no evident influence of gender or BMI.