The utility of measuring salivary cortisol has become increasingly appreciated 
since the early 1980s. Salivary cortisol is a measure of active free cortisol 
and follows the diurnal rhythm of serum or plasma cortisol. The saliva sample 
may be collected by drooling or through the use of absorbent swabs which are 
placed into the mouth until saturated. Salivary cortisol is therefore convenient 
for patients and research participants to collect noninvasively on an outpatient 
basis. Several assay techniques have been used to measure salivary cortisol, 
including radioimmunoassay and more recently liquid chromatography-tandem mass 
spectrometry. The analytical sensitivity varies between these assay methods, as 
does the potential for cross-reactivity with other steroids. The interpretation 
of salivary cortisol levels relies on rigorous standardization of sampling 
equipment, sampling protocols and assay technology with establishment of a local 
reference range. Clinically, the commonest use for salivary cortisol is 
measuring late-night salivary cortisol as a screening test for Cushing's 
syndrome. Several studies have shown diagnostic sensitivities and specificities 
of over 90%, which compares very favourably with other screening tests for 
Cushing's syndrome such as the 24-h urinary-free cortisol and the 1-mg overnight 
dexamethasone suppression test. There are emerging roles for the use of salivary 
cortisol in diagnosing adrenal insufficiency, particularly in conditions 
associated with low cortisol-binding globulin levels, and in the monitoring of 
glucocorticoid replacement. Finally, salivary cortisol has been used extensively 
as a biomarker of stress in a research setting, especially in studies examining 
psychological stress with repeated measurements.