Salivary cortisol and cortisone by LC–MS/MS: validation, reference intervals and diagnostic accuracy in Cushing's syndrome

The Endocrine Society recommends late-night salivary cortisol (LNS-F) as a first-line screening test for Cushing's syndrome (CS). In the parotid gland, 11β-hydroxysteroid dehydrogenase type 2 inactivates cortisol (F) to cortisone (E), a known source of interference in the more frequently used immunoassays. A highly specific method is mandatory in determining salivary F and E: it is widely accepted that liquid chromatography–tandem mass spectrometry (LC–MS/MS) is the best available technique for this purpose. A LC–MS/MS method with SPE of saliva samples was developed and validated. Appropriate awakening and bedtime reference ranges were established. The diagnostic performance for F, E and the ratio at bedtime was evaluated in 25 cases of CS. The method was linear, with up to 55.4 nmol/L and 51.0 nmol/L, LLOQ of 0.51 nmol/L and 0.55 nmol/L, for F and E, respectively. Within-run and between-run imprecisions were < 10% for both analytes. No ion suppression was observed. A cut-off of 2.4 nmol/L for LNS-F yielded a sensitivity of 100% and a specificity of 98% in the diagnosis of CS. The analytical performance of this method justifies its introduction into clinical practice, thus allowing clinicians the opportunity to further investigate CS and other endocrine diseases.