What is the best assessment of late-night salivary cortisol used for?

The best assessment of late-night salivary cortisol is primarily used to screen for Cushing's syndrome. This diagnostic method is grounded in the understanding that in individuals without Cushing's syndrome, cortisol levels typically drop significantly during the night, making late-night measurements an effective indicator. In patients with Cushing's syndrome, however, there is a disruption in the normal circadian rhythm of cortisol secretion, and late-night salivary cortisol levels tend to be elevated.

This method of assessment is advantageous because it is non-invasive, easy to perform, and reflects the free cortisol levels in the saliva, giving a more accurate depiction of the body's cortisol production at a time when levels should be low. High late-night salivary cortisol levels can thus indicate the presence of excessive cortisol production, warranting further diagnostic workup for Cushing's syndrome.

In contrast, assessments for chronic stress, adrenal insufficiency, or kidney function would not typically utilize late-night salivary cortisol as the primary diagnostic tool, as these conditions require different evaluation strategies and measurements.