Reliability of three versus five saliva sampling times for assessing the cortisol awakening response

The cortisol awakening response (CAR) describes the sharp increase in cortisol secretion within 60 min after awakening. A summary of the CAR, the area under the cortisol curve above the awakening cortisol value (AUC_i) is a widely used biomarker in health research. Estimation of the AUC_i rely on a number of collected salivary samples at fixed time intervals (i.e., 5 samples in 15 min intervals) starting from awakening. Little empirical work has been executed to investigate the impact of reducing sampling times on AUC_i estimation, which could potentially improve participant compliance and reduce operational costs. This study aimed to assess the reliability and validity of using 3-sample AUC_i versus 5-sample AUC_i, i.e., systematic and random fluctuations based on a large dataset from healthy and case individuals (total n = 537). We showed that the ideal timing of 3-sampling times was 0-30-60 min with a median difference in AUC_i of - 8 nmol*h/L and interquartile range of 65 nmol*h/L among healthy individuals, and - 12 nmol*h/L and 78 nmol*h/L among case individuals. We subsequently validated the 3-sample AUC_i by re-analyzing three published association studies. Overall, we obtained similar p-values with 3-sample AUC_i when compared to 5-sample AUC_i, while smaller effect sizes and standard errors were observed. In conclusion, despite a less precise estimation of the AUC_i itself, our data support that the AUC measure of the CAR, based on three samples collected at 0-30-60 min from awakening, provides reliable results in association studies.

Salivary cortisol in healthy dogs: a randomized cross-over study to evaluate different saliva stimulation methods and their effects on saliva volume and cortisol concentration

BACKGROUND

Salivary cortisol collected at home is a useful test to diagnose and monitor Cushing's syndrome in humans. The main problem in dogs is to retrieve a sufficient amount of saliva. The aim of this study was to evaluate different salivary collection methods and compare their effects on volume, pH and cortisol concentration of saliva. Sixteen healthy Beagles were used in a 4 × 4 randomized crossover study with a washout period of 1 week between each of the following collection methods: 1. Salimetrics® cotton swab dipped in ginger powder (ginger group); 2. beef-flavored Salimetrics® (bouillon group); 3. Salivette® cotton swab with an enclosed treat (treat group); 4. plain Salimetrics® (control group). First, baseline saliva (plain cotton swab, S0) and, 2 min later, experimental saliva (according to group allocation above, SExp) were collected. Saliva was gathered by holding the swabs in the animal's mouth for 2 min. After the cross-over study, another saliva sample was collected from all dogs by the ginger method, using a 30 s sampling time (30s-ginger method). Cortisol concentrations were measured by liquid chromatography tandem mass spectrometry.

RESULTS

All three stimulation methods increased saliva production significantly (S0 compared to SExp: ginger p = 0.0005; bouillon p = 0.009; treat p = 0.007). Only ginger stimulation, however, generated a significantly higher amount of saliva (SExp) compared to the control group (p = 0.00001; median (range) amount of saliva for SExp: ginger 1200 ul (600-1700), bouillon 650 ul (200-1900), treat 700 ul (300-1000), control 400 ul (0-1100)). The amount of saliva retrieved by the 30s-ginger method was still higher than that from the control group (p = 0.0004). Bouillon and treat stimulation led to decreased pH values (bouillon, p = 0.0028; treat, 0.0018). Excitement was higher in the ginger group (p = 0.01). Chewing was intensified in the ginger and treat group (ginger, p = 0.003; treat, 0.0009). The cortisol concentration SExp was higher compared to that of S0 in the ginger and treat group (p = 0.02, 0.003). The experimental cortisol concentrations (SExp) were not different between groups.

CONCLUSIONS

The 30s-ginger method could prove useful in evaluating or monitoring dogs with Cushing's syndrome, as sampling at home for 30 s by the owner seems feasible.

Salivary cortisol as a non-invasive window on the brain

The validation of accurate and meaningful assessment of cortisol in saliva samples has proved revolutionary in stress research. Its many advantages have expanded the scope of investigation from traditional laboratory and clinical settings to include multidisciplinary and community-based research. These developments have given rise to a wealth insight into the links between stress and health. Here we highlight the potential of salivary cortisol as both a product and mediator of brain function, instrumental in disturbing brain health. However, the subtleties of salivary cortisol as a measure can be underestimated, leading to misinterpretation of findings. These issues are explored, with a particular emphasis on necessary methodological rigor. Notwithstanding great promise, there is undeniably more to learn so we conclude by making recommendations for future research including use of salivary cortisol in the development of integrative predictive models of stress-related risk factors and resilience across the life course.