Determination of salivary cortisol by liquid chromatography-tandem mass spectrometry

Background: Late evening salivary cortisol concentrations are increasingly used as a screening test in suspected Cushing's syndrome partly because of easy sample collection. The cortisol immunoassays are prone to interference by cross-reacting steroids and therefore there is a need for improvement. The high specificity of an LC-MS assay provides a solution to the problem. Methods: Our liquid chromatography-tandem mass spectrometric (LC-MS/MS) analysis utilizes only 0.1 ml of saliva. The samples were extracted with dichloromethane. The extract was evaporated to dryness and cortisol was analysed by LC-MS/MS operating in the negative mode ESI after separation on a reversed-phase column. Results: The calibration curves for analysis of salivary cortisol exhibited consistent linearity and reproducibility in the range of 0.5-20 nmol/L. Interassay CVs were 4.3-11% at cortisol concentrations of 0.6-14 nmol/L. The lower limit of quantitation (LOQ) was 70 pmol/L (signal to noise ratio 10). The mean recovery of the analyte added to saliva samples ranged from 95-106%. The upper limit of the reference range (95%) was 3.0 nmol/L. Conclusions: Our method is rapid, sensitive and simple to perform with a routine LC-MS/MS spectrometer.