Endocrine Abstracts

Objective: The low dose overnight dexamethasone suppression test (1 mg DST) is a sensitive screening test for Cushing’s syndrome. The specificity of the test can be improved by simultaneous analysis of plasma dexamethasone concentrations. A simple and accurate alternative to analysis of plasma cortisol is analysis of salivary cortisone or cortisol following 1 mg DST [1]. We evaluated if analysis of salivary dexamethasone following 1 mg DST could further improve the specificity of the test. We also evaluated the stability of cortisone and cortisol in saliva samples stored 6–8 years in −80°C.

Design and methods: Saliva samples from 131 volunteers were collected with Salivette^® Cortisol at 0800 h with simultaneous collection of plasma following 1 mg DST, between March 2015 and April 2016. Saliva samples were frozen at −20°C for at least 24 h and then thawed, centrifuged, aliquoted, and stored at −80°C. One aliquot of saliva was analysed for cortisone and cortisol with liquid chromatography tandem mass spectrometry (LC–MS/MS) in April–May 2016 and a second aliquot was analysed in November 2022 with the same LC–MS/MS method, slightly modified to also include dexamethasone. Plasma dexamethasone was analysed using a different LC–MS/MS assay. A lower reference limit (LRL) for salivary dexamethasone was defined as the non-parametric 2.5th percentile of all adequately suppressed saliva samples (cortisone <3.5 nmol/l [1]) after excluding outliers using Dixon’s Q test (n=3), as recommended by the Clinical Laboratory Standards Institute. Spearman’s rank correlation coefficients (r_s) were used to test associations between hormone concentrations.

Results: The LRL for salivary dexamethasone was calculated to 0.30 nmol/l (90% CI: 0.20–0.42). Among the samples with unsuppressed salivary cortisone following 1 mg DST (>3.5 nmol/l), four out of five had a plasma dexamethasone concentration below the LRL of 3.3 nmol/l. In contrast, none had a salivary dexamethasone concentration below the LRL of 0.30 nmol/l,. Salivary and plasma dexamethasone correlated moderately (r_s=0.56). There was a high correlation between saliva samples measured in 2022 and 2016 for both cortisone and cortisol (r_s=0.98 and 0.81, respectively).

Conclusions: Analysis of salivary dexamethasone does not reflect adequate circulating dexamethasone concentrations following 1 mg DST, potentially due to enzymatic conversion of dexamethasone in the salivary gland or tablet residuals in the oral cavity. Further studies are needed on alternative dexamethasone metabolites in saliva. Salivary cortisone and cortisol concentrations are essentially unaltered after longterm storage in −80°C.

Reference: 1. Bäcklund N, et al. Eur J Endocrinol. 2020. PMID: 32213657