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Salivary cortisol reflects the biologically active form of serum cortisol, offering a noninvasive evaluation method for the diurnal rhythm of the hypothalamic–pituitary–adrenal (HPA) axis. While liquid chromatography–tandem mass spectrometry (LC–MS/MS) is known for its specificity, immunoassays (IA) are commonly used because of their simplicity. This study aimed to assess the performance of salivary cortisol measurement using both IA and LC–MS/MS in comparison to serum free cortisol measurement. and Methods: Assay results for 188 saliva and 94 serum samples from 47 participants were analyzed. Salivary samples collected at different time points were analyzed using IA and LC–MS/MS. Serum samples were analyzed for cortisol, cortisol-binding globulin, and serum free cortisol. The statistical analyses included correlations and method comparisons. The diurnal salivary cortisol profiles exhibited a comparable circadian rhythm pattern; however, the concentrations measured using IA were consistently higher than those measured using LC–MS/MS. The correlation analysis revealed robust associations among salivary cortisol (IA), salivary cortisol (LC–MS/MS), and serum free cortisol levels (LC-MS/MS). However, the method comparison revealed a systematic bias between IA and LC–MS/MS in salivary cortisol measurement. This study contributes to the ongoing debate on assay techniques by affirming the suitability of IA and LC–MS/MS for salivary cortisol measurement to assess dynamic changes in HPA axis activity. The identified systematic bias emphasizes the importance of selecting methods based on specific research or clinical requirements.
Lee et al. (2024) studied this question.