Establishment and Clinical Application of Reference Intervals of Salivary Cortisol

Saliva testing is a non-invasive alternative to blood tests for measuring cortisol, a key hormone for assessing endocrine status and diagnosing conditions like Cushing's syndrome. It accurately reflects physiologically active free cortisol. However, its clinical use is hampered by extremely low cortisol concentrations and the lack of reliable detection methods. We have developed two advanced technologies: an immunomagnetic-bead-based LC-MS assay with simplified preparation, and a fluorescence immunoassay system for point-of-care testing. To promote their clinical adoption, this study will establish normal reference intervals of salivary cortisol for both methods in a healthy population and optimal cut-offs in cases of Cushing's sydrome and adrenal insufficiency.

Healthy Volunteers:

Inclusion criteria

• Age ≥ 18 and ≤ 60 years old;
• Body mass index (BMI) ≥ 18.5 and ≤ 24.9 kg*m^-2;
• No previous history of chronic diseases such as hyperglycemia, hypertension, dyslipidemia, coronary heart disease and stroke;
• Normal glucose regulation, defined as: fasting blood-glucose < 5.6 mmol/L, 2-hour blood-glucose after glucose load < 7.8 mmol/L, and glycated hemoglobin (HbA1c) < 5.7%.

Exclusion criteria

• Liver or kidney dysfunction, alanine aminotransferase (ALT) or aspartate aminotransferase (AST) or direct bilirubin higher than 1.5 times the upper limit of normal, or serum creatinine >115 μmol/L;
• Pregnancy or lactation women, or people with cancer or mental illness;
• Factors affecting cortisol levels, including hypothalamus-pituitary-adrenal axis disease, autoimmune disease (systemic lupus erythematosus, rheumatoid arthritis, etc.), diagnosed mental disease, Alzheimer's disease, alcoholism (alcohol > 60 g/d for male, >40 g/d for women) and corticosteroid therapy in the past 3 months (females with contraceptives or estrogen);
• Factors affecting saliva collection, such as serious oral problems (oral ulcers, gingival bleeding);
• Night shift workers, who are awake from 11:00 PM to 7:00 AM;
• Acute infection (body temperature ≥ 37.3 ℃ or C-reactive protein > 50 mg/L).

Cases:

Inclusion criteria

• Aged 18 to 80 years old;
• Patients with suspected Cushing's syndrome or adrenal insufficiency.

Exclusion criteria

• Corticosteroid therapy within the last 6 weeks;
• Acute infection;
• Severe oral conditions;
• Severe liver and kidney dysfunction;
• Alcoholism, depression, or other psychiatric disorders.